DR IVING-TESTS. ORG
THE TOP 80 MOST COMMON Air brakes
QUESTIONS
Lose the fear of the unknown. Study the most common
CDL air brakes questions so you’ll know what to expect
on the big test day.
Know what
to expect
DRIVING-TESTS. ORG
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documents into the knowledge
exam room to cheat with
The term used to mean a sheet of notes that you kept with you to
cheat on an exam. Today, it has also come to mean a quick reference or ready
reference of the most important or frequently used items. That’s how we intend
our “cheat sheets” to be used: As quick references for you to browse to help
you study for your written knowledge test.
“cheat sheet”
1. If you do not have an automatic tank drain,
when should you drain your air tanks?
At the end of each working day.
Oil and water can start to collect in the bottom
of an air tank and can cause failure if it’s not
drained. Unless your vehicle automatically
drains the air tanks, you should drain them at
the end of each working day so the moisture
and oil will not freeze or coagulate and cause
your brakes to fail.
2. To check the slack adjusters on S-cam
drum brakes, you should first
park on level ground, chock the wheels, and
release the parking brakes.
Chock the wheels so you can release the
parking brakes safely. Then put on gloves and
pull hard on each slack adjuster you can reach.
If it moves more than about one inch, it needs
to be adjusted.
3. The safety valve is set to automatically
reduce pressure at
150 psi.
The safety valve, which protects the system
from excessive pressure, is usually set to open
when the system reaches 150 psi.
4. The driver must be able to see a warning
before air pressure in the service air tanks
falls below
55 to 60 psi.
A low pressure warning signal visible to the
driver must come on before the pressure in the
service air tank falls below 55 psi (or one half
the air compressor governor cutout air pressure
on older vehicles).
5. A typical air brake system is fully charged at
125 psi.
A typical air brake system is fully charged at 125
psi. Check the manufacturer’s specifications for
the actual figure for your vehicle.
6. During normal driving, spring brakes are
usually held back by
air pressure.
Spring brakes contain powerful springs that are
held back by air pressure. If the pressure drops too
low, the spring brakes will automatically kick in.
7. Excessive use of the service brakes may
result in overheating, which can lead to
expansion of the brake drums.
Overusing the service brakes can cause
overheating and expansion of the brake drums.
The brake shoes will have to move farther to
contact the brake drums, making braking
harder or eventually impossible (brake fade).
8. If your vehicle has a properly functioning
dual air brake system and minimum-sized air
tanks, the air pressure should build from 85
to 100 psi within _________ seconds.
45
In dual air systems with minimum sized air
tanks, the pressure should build from 85 to 100
psi within 45 seconds. (With larger air tanks, it
can take longer. Check the manufacturer’s
specifications for the expected amount of time.)
If it takes longer than it should, there may be a
problem with your system that may cause you
to lose air pressure while you’re driving.
9. Repeatedly pressing and releasing
(fanning) the brake pedal may result in
a loss of brake air pressure.
Each time you release the brakes, some
compressed air leaves the system and must be
replenished by the air compressor. If you keep
pressing and releasing the brake pedal, air may
leave the system faster than the air compressor
can replenish it. The air pressure may drop to
the point that the brakes won’t work.
The top 80 most common AIR brakes questions 1
10. The total stopping distance for air brakes
is longer than that for hydraulic brakes
because of
brake lag.
When you depress the brake pedal on a vehicle
with air brakes, there will be a lag of about a
half second before the air flows through the
lines to reach the brakes. This will add to your
total stopping distance.
11. At 55 mph on dry pavement, the air brake
lag distance is about
32 feet.
When you depress the brake pedal, it will take
perhaps a half second for the air to reach the
brakes. At 55 mph, your vehicle will travel about
32 feet in that time before the brakes kick in.
This will add to your total stopping distance.
12. When you apply the brake pedal, the brake
shoes and linings are pressed against the
brake drum.
In North America, the most common type of
foundation brake is the S-cam drum brake. The
brake shoes and linings press against the inside
of the brake drum, which causes friction to slow
the wheel.
13. On a long or steep downgrade, once you
have reached your “safe” speed, brake until
you are traveling
5 mph slower.
On a steep downgrade, don’t keep applying your
brakes continuously because this might cause
them to fade (become less effective). Instead,
apply your brakes sparingly as a supplement to
engine braking. Select a “safe” speed. Let your
vehicle reach this speed. Brake gently until you
are going 5 mph slower. Then let your speed
increase back to the “safe” speed. Keep varying
your speed up and down this way.
14. Air braking takes more time than
hydraulic braking because air
takes more time to flow through the lines than
hydraulic fluid.
With hydraulic brakes, hydraulic fluid is always
in the system, so the brakes will be applied
instantly when you depress the brake pedal.
With air brakes, the compressed air must flow
through a complex series of lines, valves, etc.,
to reach and fill the brake chambers before the
brakes can begin to slow your vehicle. This
“brake lag” can take at least a half second.
15. On large, heavy vehicles, the parking or
emergency brakes must be held on by
something that cannot leak away, such as
spring pressure.
Federal safety regulations require buses and
trucks to have parking and emergency brakes
that don’t rely on compressed air or hydraulic
fluid to work (because these can leak away).
Hence, the parking and emergency brakes rely
on mechanical force such as springs.
16. During your walk around inspection,
check the brake drums to be sure that no
cracks are longer than _________ the width
of the friction area.
one-half
During your walk-around inspection, check the
brake drums to be sure that no cracks are
longer than half the width of the friction area.
17. In the event of a brake system failure, the
emergency brake system uses _________ to
stop the vehicle.
parts of the service brakes and parking brakes
Spring brakes use powerful springs to apply the
brakes. Normally, air pressure in the brake
chambers holds back the springs. Should air
pressure drop too low because of a leak, the
springs will apply the brakes and stop the
vehicle. To apply the parking brakes, you
release air from the brake chambers manually.
The top 80 most common AIR brakes questions 2
18. What is a dual air brake system?
A two-brake system: one brake operates the
front brakes, and one operates the back
brakes.
Many heavy-duty trucks and buses have dual
air brakes, which are two sets of air brakes for
the front and back axles, but controlled by one
brake pedal.
19. To check the slack adjusters on S-cam
drum brakes, you should first
park on level ground, chock the wheels, and
release the parking brakes.
Chock the wheels so you can release the
parking brakes safely. Then put on gloves and
pull hard on each slack adjuster you can reach.
If it moves more than about one inch, it needs
to be adjusted.
20. Before starting down a hill, you should
be in the proper gear
so you only have to apply the brake just hard
enough to feel a definite slowdown.
Starting down a hill, you should put your vehicle
in a low gear and use the brakes just enough to
keep you near your safe speed.
21. The driver must be able to see a warning
before air pressure in the service air tanks
falls below
55 to 60 psi.
A low pressure warning signal visible to the
driver must come on before the pressure in the
service air tank falls below 55 psi (or one-half
the air compressor governor cutout air pressure
on older vehicles).
22. With air brake-equipped vehicles, the
parking brakes should be used
whenever you leave the vehicle unattended.
Because air pressure can leak when you re
away from your vehicle, it should be set with the
parking brake, to keep it from rolling. There is
an exception to this rule: if the brakes are too
hot or wet, use wheel chocks instead, to avoid
brake damage or freezing.
23. Typically, the air compressor should start
pumping at about
100 psi.
Typically, the air compressor will start pumping
around 100 psi and stop at about 125 psi. Check
the manufacturer s specifications for the exact
values.
24. The modulating control valve allows you
to control the
spring brakes.
In some vehicles a control handle on the dash
board may be used to apply the spring brakes
gradually. This is called a modulating valve. It is
spring-loaded so you have a feel for the braking
action. The more you move the control lever,
the harder the spring brakes come on. They
work this way so you can control the spring
brakes if the service brakes fail. When parking a
vehicle with a modulating control valve, move
the lever as far as it will go and hold it in place
with the locking device.
25. The stop light switch
turns on the brake lights when you brake.
The stop light switch is an electric switch
triggered by air pressure. When you apply the
service brakes, the stop light switch turns on
the brake lights.
26. Modern air brake systems are three
different systems combined: the service
brakes, parking brakes, and _________
brakes.
emergency
The emergency brake system utilizes parts of
the service brakes and the parking brakes in
case your braking systems fail and need to
come to a stop.
27. Why must air tanks be drained?
Your brakes may fail because of water
freezing.
Your air tanks hold a mixture of water and
compressor oil. If left in to collect at the bottom,
the water may freeze and cause brake failure.
The top 80 most common AIR brakes questions 3
28. The air compressor governor controls
when air is pumped into the air storage tanks.
The air compressor runs at all times while the
engine is running. To keep the air pressure from
eventually rising too high, the air compressor
governor controls when the compressed air
from the air compressor should flow into the air
reservoirs. When air pressure drops to a certain
level (typically 100 psi), the governor “cuts in”
and routes the compressed air into the storage
tanks. When air pressure rises to a certain level
(typically 125 psi), the governor “cuts out” and
routes the compressed air out of the system,
into the atmosphere.
29. If your vehicle has an alcohol evaporator,
you should _________ daily in cold weather.
check the alcohol level and refill as needed
Alcohol vapor from the alcohol evaporator
helps keep ice from forming in the lines and
valves of the air brake system. Check the
alcohol level daily and refill as needed.
30. To drive a tractor-trailer with air brakes,
you must have
a Class A CDL without an air brakes restriction.
To legally pull a trailer whose gross vehicle
weight rating (GCWR) exceeds 10,000 pounds,
you’ll have to obtain a Class A CDL. If you don’t
pass a knowledge test on air brakes, your CDL
will include a restriction limiting you to driving
only vehicles without air brakes.
31. If the low air pressure warning signal is
not working,
it may lead to sudden emergency braking in a
single circuit air system.
The low pressure warning signal should come
on before the air pressure falls below 60 psi. If
it fails to do so, the air pressure may keep
falling without your knowing it. In a single circuit
air system, your spring (emergency) brakes may
come on suddenly. In a dual air system, either
the front or the rear brakes won’t work properly,
the braking distance of your vehicle will
increase, and continued braking will soon cause
the spring brakes to come on.
32. The air supply pressure gauge shows
how much pressure
is in the air tanks.
The air supply pressure gauge is connected to
the air tank (or tanks) and shows how much air
pressure is present.
33. You ll know that your brakes are fading if
you have to press the brake pedal harder
than usual to control your speed.
If your brakes begin to fade, you’ll notice that
you have to depress the brake pedal harder to
achieve the same braking effect as before. If
your vehicle is so equipped, check the
application pressure gauge to see just how
much pedal effort you are employing. If the
brakes continue to fade, you may not be able to
slow down or stop at all.
34. Tractor and straight truck spring brakes
will come fully on when the air pressure
drops to a range of
20 to 45 psi.
An air leak in your brake system can cause your
air brakes to fail. If your air pressure drops to
20-45 psi, your spring brakes will come on
automatically. You can avoid this by pulling over
once the low pressure light comes on so you
can address the problem.
35. Your vehicle has a dual air brake system.
One of the systems loses air pressure. What
will happen?
Either the front or rear brakes will not be fully
operational.
A dual air brake system has two separate air
brake systems, each with its own air tanks,
hoses, lines, etc. Typically, one system operates
the brakes on the rear axle(s), and the other
system operates the brakes on the front axle. If
one of the systems loses air pressure, either the
front or the rear brakes won’t work properly. As
a result, your vehicle’s braking distance will
increase. If this happens, stop your vehicle and
get the system fixed. Continued braking would
soon cause the spring brakes to come on.
The top 80 most common AIR brakes questions 4
36. If your truck or bus has dual parking
control valves, you can use air pressure from
a separate tank to
release the spring emergency/parking brakes.
With a dual parking control valve you can use
the pressure from a separate air tank to give
enough air pressure to release the spring
brakes and move your vehicle. This is very
useful in emergency situations.
37. The most common type of foundation
brake found on vehicles with air brakes is the
S-cam drum brake.
Foundation brakes are present on each wheel.
In North America as of this writing, S-cam drum
brakes are much more common than wedge or
disc brakes.
38. If you must make an emergency stop,
brake so that
your vehicle will stay in a straight line.
If you must brake in an emergency, brake in a way
that keeps your vehicle in a straight line while still
letting you turn if necessary. You can use the
“controlled braking” method or (in a vehicle without
antilock brakes) the “stab braking” method.
39. The braking system that applies the
brakes when the driver presses the brake
pedal is the _________ brake system.
service
Air brakes are really three braking systems in one:
the emergency brake, the parking brake, and the
service brake. As these names suggest, when you
depress the brake pedal in normal driving, it’s the
service brake system that applies the brakes.
40. In a vehicle equipped with air brakes, the
brake pedal
controls the air pressure applied to the
service brakes.
The brake pedal controls the amount of air
pressure applied to the service brakes. The
harder the pedal is pushed, the more air
pressure is applied to the brakes.
41. To apply the parking brakes under
normal conditions,
let the air out of the brake chambers.
The parking brake control allows you to
evacuate air from the brake chambers. With the
air gone, the spring brakes will come on
automatically and hold your vehicle in place.
42. Air tanks must be drained
to drain moisture and oil out of the tanks.
Water and oil from the compressor can collect
in the air tanks and must be drained daily to
prevent brake failure.
43. What are spring brakes?
If the air brakes leak down, springs apply the
brakes to stop the vehicle.
Springs brakes will automatically come on if air
pressure in the tanks drop too low and will not
be able to stop the vehicle. They are used as
emergency brakes and parking brakes.
44. The use of air brakes on a downgrade is
only a supplement to the
braking effect of the engine.
Going down a hill, you should keep your engine in
a low gear to keep your speed low, and use the
air brakes as a supplemental tool to slow down.
45. If your truck or bus has dual parking
control valves, you can use air pressure from
a separate tank to
release the spring brakes.
Even after a braking problem has caused the
spring brakes to come on and stop your vehicle,
you may need to move the vehicle again
perhaps to get it out of immediate danger.
Some vehicles such as buses have a separate
air tank that can be used to release the spring
brakes. You can then move the vehicle, after
which you can set the spring brakes again. This
action is controlled by dual parking control
valves: One valve lets air flow from this air tank
to release the spring brakes, and the other
valve sets them again.
The top 80 most common AIR brakes questions 5
46. How do you adjust a manual slack adjuster?
Turn the adjusting nut to move the end of the
slack adjuster until the brake shoes contact
the inside of the brake drum. Then turn the
nut the other way to move the brake shoes
away from the brake drum.
To adjust a manual slack adjuster, turn the
adjusting nut so that the S-cam rotates in the
same direction as when the brakes are applied.
Keep turning the nut until the nut stops turning.
At this point, the brake shoes should be
pressing tight against the inside of the brake
drum. Then turn the nut the other way about
one-third of a turn to move the brake shoes
away from the inside of the brake drum.
47. Every vehicle equipped with air brakes
must have
a supply pressure gauge.
Every vehicle with air brakes has a supply
pressure gauge, which shows how much air
pressure is in the air tanks. Some vehicles also
have an application pressure gauge, which shows
how much air pressure you are applying to the
brakes. Older vehicles also had a manual front
brake limiting valve, which allowed the driver to
cut front brake air pressure in half as a way of
preventing front-wheel skids. However, tests have
shown that full front-wheel braking is safe.
48. With air brake-equipped vehicles, the
parking brakes should be used
whenever you leave the vehicle unattended.
Because air pressure can leak when you’re
away from your vehicle, it should be set with the
parking brake, to keep it from rolling. There is
an exception to this rule: if the brakes are too
hot or wet, use wheel chocks instead, to avoid
brake damage or freezing.
49. If oil and water collects in the air tanks,
what could happen to the brakes?
The brakes could fail.
Air tanks must be drained daily to make sure
there is no accumulation of water or oil than can
build up and cause the brakes to fail.
50. When can you leave your truck
unattended without first applying the
parking brakes or chocking the wheels?
Never.
Never leave your vehicle unattended without
first applying the parking brakes or chocking
the wheels. Your vehicle might roll away and hit
something or someone.
51. Which of the following can cause brakes
to fail or fade?
Overheating, low air pressure, and not relying
on the engine braking effect
Low air pressure causes your service brakes to
fail, using brakes that have overheated can
cause damage resulting in brake failure, and
having brakes gradually out of alignment can
cause brake fade.
52. To test the static air leakage rate, you should
turn off the engine, release the parking brake,
and let the system settle.
To test for an air leak, turn off the engine,
release the parking brake (i.e., push the parking
brake control knob in), and see how fast the air
pressure drops. The air pressure should drop
less than 2 psi in one minute for a single vehicle
and less than 3 psi for a combination vehicle.
53. As part of your vehicle inspection test, if
your vehicle is equipped with air brakes and has
a trailer, you will inspect the air connections
between the truck or tractor and the trailer.
Make sure that the _________ are locked in
place and free of damage or air leaks.
glad hands
As part of your vehicle inspection test, if your
vehicle is equipped with air brakes and has a trailer,
you will inspect the air connections between the
truck or tractor and the trailer. These connections
are made with hose couplers (commonly called
glad hands because they vaguely resemble
shaking hands). These glad hands have rubber
seals to form a tight connection and prevent air
from escaping. Make sure that the glad hands are
locked in place and free of damage or air leaks.
The top 80 most common AIR brakes questions 6
54. To correct a drive-wheel braking skid,
you should
stop braking, turn quickly, and countersteer.
To recover from a drive-wheel braking skid, first
stop braking. The rear wheels will unlock and
roll again so there will be no more tendency to
slide sideways. Then quickly steer your vehicle
back on course. You may find that your vehicle
will overshoot and keep on turning, possibly
skidding in the opposite direction. Countersteer
to counteract this. Note: This maneuver takes
some practice. You should become proficient in
it before you actually need to use it.
55. The application pressure gauge shows
the amount of air pressure
that you are applying to the brakes.
If your vehicle is so equipped, the application
pressure gauge will show how much air pressure
you are applying to the brakes. If you have to
apply more pressure than before to achieve the
same braking effect, your brakes are fading.
56. You are on a steep downgrade and have
reached your safe speed of 40 mph. You
would apply the service brakes until your
speed drops to
35 mph.
When driving down a decline, use your brakes
to keep you near your safe speed. Press the
brakes to slow your vehicle to 5 mph below
your safe speed and then release; repeat as
needed once you surpass your safe speed.
57. Before you drive a vehicle with a dual air
brake system, wait until the pressure in each
system reaches at least
100 psi.
Before you drive a vehicle with a dual air brake
system, wait until the air compressor has raised
the air pressure to at least 100 psi in both the
primary and secondary systems. Watch the
primary and secondary supply pressure gauges
(or needles, if the system has two needles in
one gauge).
58. Some air brake systems have an alcohol
evaporator. What may happen if you don t
keep the unit filled with alcohol?
Ice may form in the air storage tanks and
cause the brakes to fail.
Moisture in the warm, compressed air from the air
compressor will condense as the air cools. In
below-freezing temperatures, this moisture can
freeze. Ice in the tanks and air lines can cause the
brakes to stop working. Whenever the air
compressor runs, the alcohol evaporator puts
alcohol vapor into the air brake system to help
prevent water in the lines or valves from freezing.
(Alcohol has a lower freezing point than water.)
59. How should you check that the spring
brakes come on when the air pressure in the
system drops below a certain level?
Park on level ground, chock the wheels,
release the parking brake when you have
enough air pressure, shut the engine off, and
repeatedly press and release the brake pedal.
When doing your final check of your air brake,
you should cause your vehicle to have low air
pressure, to be sure the warning signal comes
on, by turning the engine off, with the wheels
chocked, and pushing on and off the brakes to
release the air pressure. Once you re sure the
low pressure warning is viable, continue doing
the same until the spring brakes come on
automatically, usually between 20- 45 psi.
60. To make an emergency stop with the
stab braking method, you should
depress the brake pedal as hard as you can,
release the brakes when the wheels lock, and
apply the brakes again when the wheels start
to roll.
To stop with stab braking, brake hard, release
the brakes once the wheels lock, and then
brake hard once the wheels start to roll again.
(Clearly, this method won t work if your vehicle
has antilock brakes, because the wheels won t
lock. Use controlled braking instead.)
The top 80 most common AIR brakes questions 7
61. If the air compressor should develop a
leak, what will keep the air in the tanks?
The one-way check valve
Federal regulations require a one-way check
valve or an equivalent device to be located at
the entrance to the first air tank. (In a dual air
brake system, there must be a one-way check
valve at the entrance to both the primary and
secondary air tanks.) If the air compressor or
the line connecting it to the air tank develops a
leak, the one-way check valve will prevent the
air from going out.
62. The braking power of the spring brakes
depends on whether the service brakes are in
proper adjustment.
The braking power of the spring brakes
depends on whether the service brakes are in
proper adjustment. If they are not, neither the
service brakes nor the emergency/parking
brakes will work properly.
63. Excessive heat caused by overuse of the
service brakes can cause
the brakes to fade.
Excessive heat caused by overuse of the
brakes can expand the brake drums away from
the brake pads or shoes and affect the brake
lining, reducing friction. The brakes will fade
(lose their effectiveness). On a long or steep
downgrade, rely on engine braking more than
the service brakes.
64. When you pull hard on a slack adjuster, if
it has more than _________ of free play, the
slack adjuster may need adjustment.
1 inch
During the walk-around inspection, pull hard on
each slack adjuster that you can reach. If it moves
more than an inch, it probably needs adjustment.
65. Air brakes use _________ to make the
brakes work.
compressed air
Air brakes use compressed air to make the
brakes work.
66. If the spring brakes are on, when should
you push the brake pedal?
Never.
Applying the spring brakes and the air brakes at
the same time may be too much force on the
brakes and can actually damage them.
67. What is the first thing you should do if
the low air pressure warning comes on?
Stop.
If the low air pressure warning comes on, there
may be a leak in the system. Pull over and park
your vehicle as soon as possible. If the air
pressure drops too low, the spring brakes will
come on automatically, which will stop your
vehicle but not necessarily in a safe way.
Sudden application of the spring brakes while
you re driving might result in a skid.
68. During your final air brake check, if the
air pressure does not build up fast enough,
the air pressure may drop too low during driving.
The manufacturer s specifications should state
how fast the air pressure should build up once
the engine has reached normal operating
speed. If the air pressure does not build up fast
enough during your final air brake check, the air
pressure may not replenish quickly enough
during normal braking either. Don t drive the
vehicle until the cause has been found and
corrected.
69. The S-cam
forces the brake shoes against the inside of
the brake drum.
When the S-cam rotates, it forces the brake
shoes apart and against the inside of the brake
drum. Friction will slow the rotation of the wheel.
70. The parking brake control
lets the air out of the brake chambers.
When you apply the parking brake control, air is
released from the brake chambers, causing the
springs to apply the brakes and hold your
vehicle in place.
The top 80 most common AIR brakes questions 8
71. Under ideal conditions, the average
driver of a truck or bus equipped with air
brakes and traveling at 55 mph would
require what stopping distance?
More than 400 feet
The stopping distance equals the sum of the
perception distance, reaction distance, brake
lag distance, and braking distance. Under ideal
conditions, the average driver of a bus or large
truck equipped with air brakes and traveling at
55 mph would require about 451 feet to come
to a complete stop. The length of a football field
including the two end zones is 360 feet.
72. With S-cam drum brakes, when the brake
chamber fills with air, air pressure pushes
the push rod out, moving the _________
and thus rotating the _________.
slack adjuster; brake camshaft
To apply the S-cam drum brakes, air is let into
each brake chamber. Air pressure pushes the
push rod out, moving the slack adjuster, thus
rotating the brake camshaft. As the S-cam on
the camshaft rotates, it forces the brake shoes
against the inside of the brake drum.
73. What is the best way to test your
vehicle’s low air pressure warning signal?
With the engine off, step on and off the brake
pedal to lower the air pressure below 60 psi.
The best way to test the low air pressure warning
signal is to release the parking brake (all
vehicles), release the tractor protection valve (on
combination vehicles), and turn off the engine of
your vehicle while leaving the electrical power
on. Then step on and off the brake pedal. The
low air pressure warning signal should come on
when the pressure falls below 60 psi. You should
never need to get down to 30 psi.
74. To test the air service brakes,
brake firmly while slowly moving forward.
To test the service brakes, wait for normal air
pressure. Then move your vehicle forward
slowly (at about 5 mph) and brake hard. Check
whether your vehicle stops promptly without
pulling to one side or the other.
75. What is the purpose of an alcohol evaporator?
To prevent a buildup of ice in the air brake system.
Alcohol evaporators put alcohol vapor into the
air system to help prevent water in the lines or
valves from freezing. Ice in the lines or valves
can cause them to stop working.
76. S-cam drum brakes have an S-cam in
each brake. Why is it called an S-cam?
It is shaped like the letter S .
The S-cam really is S-shaped. As it rotates, it
forces the brake shoes away from each other
and toward the inside of the brake drum.
77. A converter dolly with antilock brakes
(ABS) is required to have
a yellow lamp on the left side.
A converter dolly with antilock brakes is
required to have a yellow ABS lamp on its left
side. All converter dollies built since March 1,
1998 have antilock brakes.
78. Which of the following is the most
important thing about hard braking?
Don t lock the wheels for longer than an instant.
When braking hard, don t lock the wheels for
longer than an instant (which is done in stab
braking). If the wheels stay locked, they will lose
the grip of the road and your vehicle may skid
or even jackknife.
79. In controlled braking, you
brake in a straight line.
In controlled braking, you brake as hard as you
can without locking the wheels. As you do this,
keep your vehicle going relatively straight.
80. The air loss rate for a straight truck or
bus with the engine off and the brakes on
should not be more than
3 psi in one minute.
When testing for an air leak, you should turn off
the engine and hold the brake for one minute:
the air pressure should not drop more than 3
psi in that one minute.
The top 80 most common AIR brakes questions 9
- If you do not have an automatic tank drain,
when should you drain your air tanks?
At the end of each working day.
Oil and water can start to collect in the bottom
of an air tank and can cause failure if it’s not
drained. Unless your vehicle automatically
drains the air tanks, you should drain them at
the end of each working day so the moisture
and oil will not freeze or coagulate and cause
your brakes to fail. - To check the slack adjusters on S-cam
drum brakes, you should first
park on level ground, chock the wheels, and
release the parking brakes.
Chock the wheels so you can release the
parking brakes safely. Then put on gloves and
pull hard on each slack adjuster you can reach.
If it moves more than about one inch, it needs
to be adjusted. - The safety valve is set to automatically
reduce pressure at
150 psi.
The safety valve, which protects the system
from excessive pressure, is usually set to open
when the system reaches 150 psi. - The driver must be able to see a warning
before air pressure in the service air tanks
falls below
55 to 60 psi.
A low pressure warning signal visible to the
driver must come on before the pressure in the
service air tank falls below 55 psi (or one half
the air compressor governor cutout air pressure
on older vehicles). - A typical air brake system is fully charged at
125 psi.
A typical air brake system is fully charged at 125
psi. Check the manufacturer’s specifications for
the actual figure for your vehicle. - During normal driving, spring brakes are
usually held back by
air pressure.
Spring brakes contain powerful springs that are
held back by air pressure. If the pressure drops too
low, the spring brakes will automatically kick in. - Excessive use of the service brakes may
result in overheating, which can lead to
expansion of the brake drums.
Overusing the service brakes can cause
overheating and expansion of the brake drums.
The brake shoes will have to move farther to
contact the brake drums, making braking
harder or eventually impossible (brake fade). - If your vehicle has a properly functioning
dual air brake system and minimum-sized air
tanks, the air pressure should build from 85
to 100 psi within _ seconds.
45
In dual air systems with minimum sized air
tanks, the pressure should build from 85 to 100
psi within 45 seconds. (With larger air tanks, it
can take longer. Check the manufacturer’s
specifications for the expected amount of time.)
If it takes longer than it should, there may be a
problem with your system that may cause you
to lose air pressure while you’re driving. - Repeatedly pressing and releasing
(fanning) the brake pedal may result in
a loss of brake air pressure.
Each time you release the brakes, some
compressed air leaves the system and must be
replenished by the air compressor. If you keep
pressing and releasing the brake pedal, air may
leave the system faster than the air compressor
can replenish it. The air pressure may drop to
the point that the brakes won’t work.
The top 80 most common AIR brakes questions 1 - The total stopping distance for air brakes
is longer than that for hydraulic brakes
because of
brake lag.
When you depress the brake pedal on a vehicle
with air brakes, there will be a lag of about a
half second before the air flows through the
lines to reach the brakes. This will add to your
total stopping distance. - At 55 mph on dry pavement, the air brake
lag distance is about
32 feet.
When you depress the brake pedal, it will take
perhaps a half second for the air to reach the
brakes. At 55 mph, your vehicle will travel about
32 feet in that time before the brakes kick in.
This will add to your total stopping distance. - When you apply the brake pedal, the brake
shoes and linings are pressed against the
brake drum.
In North America, the most common type of
foundation brake is the S-cam drum brake. The
brake shoes and linings press against the inside
of the brake drum, which causes friction to slow
the wheel. - On a long or steep downgrade, once you
have reached your “safe” speed, brake until
you are traveling
5 mph slower.
On a steep downgrade, don’t keep applying your
brakes continuously because this might cause
them to fade (become less effective). Instead,
apply your brakes sparingly as a supplement to
engine braking. Select a “safe” speed. Let your
vehicle reach this speed. Brake gently until you
are going 5 mph slower. Then let your speed
increase back to the “safe” speed. Keep varying
your speed up and down this way. - Air braking takes more time than
hydraulic braking because air
takes more time to flow through the lines than
hydraulic fluid.
With hydraulic brakes, hydraulic fluid is always
in the system, so the brakes will be applied
instantly when you depress the brake pedal.
With air brakes, the compressed air must flow
through a complex series of lines, valves, etc.,
to reach and fill the brake chambers before the
brakes can begin to slow your vehicle. This
“brake lag” can take at least a half second. - On large, heavy vehicles, the parking or
emergency brakes must be held on by
something that cannot leak away, such as
spring pressure.
Federal safety regulations require buses and
trucks to have parking and emergency brakes
that don’t rely on compressed air or hydraulic
fluid to work (because these can leak away).
Hence, the parking and emergency brakes rely
on mechanical force such as springs. - During your walk around inspection,
check the brake drums to be sure that no
cracks are longer than _ the width
of the friction area.
one-half
During your walk-around inspection, check the
brake drums to be sure that no cracks are
longer than half the width of the friction area. - In the event of a brake system failure, the
emergency brake system uses _ to
stop the vehicle.
parts of the service brakes and parking brakes
Spring brakes use powerful springs to apply the
brakes. Normally, air pressure in the brake
chambers holds back the springs. Should air
pressure drop too low because of a leak, the
springs will apply the brakes and stop the
vehicle. To apply the parking brakes, you
release air from the brake chambers manually.
The top 80 most common AIR brakes questions 2 - What is a dual air brake system?
A two-brake system: one brake operates the
front brakes, and one operates the back
brakes.
Many heavy-duty trucks and buses have dual
air brakes, which are two sets of air brakes for
the front and back axles, but controlled by one
brake pedal. - To check the slack adjusters on S-cam
drum brakes, you should first
park on level ground, chock the wheels, and
release the parking brakes.
Chock the wheels so you can release the
parking brakes safely. Then put on gloves and
pull hard on each slack adjuster you can reach.
If it moves more than about one inch, it needs
to be adjusted. - Before starting down a hill, you should
be in the proper gear
so you only have to apply the brake just hard
enough to feel a definite slowdown.
Starting down a hill, you should put your vehicle
in a low gear and use the brakes just enough to
keep you near your safe speed. - The driver must be able to see a warning
before air pressure in the service air tanks
falls below
55 to 60 psi.
A low pressure warning signal visible to the
driver must come on before the pressure in the
service air tank falls below 55 psi (or one-half
the air compressor governor cutout air pressure
on older vehicles). - With air brake-equipped vehicles, the
parking brakes should be used
whenever you leave the vehicle unattended.
Because air pressure can leak when you re
away from your vehicle, it should be set with the
parking brake, to keep it from rolling. There is
an exception to this rule: if the brakes are too
hot or wet, use wheel chocks instead, to avoid
brake damage or freezing. - Typically, the air compressor should start
pumping at about
100 psi.
Typically, the air compressor will start pumping
around 100 psi and stop at about 125 psi. Check
the manufacturer s specifications for the exact
values. - The modulating control valve allows you
to control the
spring brakes.
In some vehicles a control handle on the dash
board may be used to apply the spring brakes
gradually. This is called a modulating valve. It is
spring-loaded so you have a feel for the braking
action. The more you move the control lever,
the harder the spring brakes come on. They
work this way so you can control the spring
brakes if the service brakes fail. When parking a
vehicle with a modulating control valve, move
the lever as far as it will go and hold it in place
with the locking device. - The stop light switch
turns on the brake lights when you brake.
The stop light switch is an electric switch
triggered by air pressure. When you apply the
service brakes, the stop light switch turns on
the brake lights. - Modern air brake systems are three
different systems combined: the service
brakes, parking brakes, and _
brakes.
emergency
The emergency brake system utilizes parts of
the service brakes and the parking brakes in
case your braking systems fail and need to
come to a stop. - Why must air tanks be drained?
Your brakes may fail because of water
freezing.
Your air tanks hold a mixture of water and
compressor oil. If left in to collect at the bottom,
the water may freeze and cause brake failure.
The top 80 most common AIR brakes questions 3 - The air compressor governor controls
when air is pumped into the air storage tanks.
The air compressor runs at all times while the
engine is running. To keep the air pressure from
eventually rising too high, the air compressor
governor controls when the compressed air
from the air compressor should flow into the air
reservoirs. When air pressure drops to a certain
level (typically 100 psi), the governor “cuts in”
and routes the compressed air into the storage
tanks. When air pressure rises to a certain level
(typically 125 psi), the governor “cuts out” and
routes the compressed air out of the system,
into the atmosphere. - If your vehicle has an alcohol evaporator,
you should _ daily in cold weather.
check the alcohol level and refill as needed
Alcohol vapor from the alcohol evaporator
helps keep ice from forming in the lines and
valves of the air brake system. Check the
alcohol level daily and refill as needed. - To drive a tractor-trailer with air brakes,
you must have
a Class A CDL without an air brakes restriction.
To legally pull a trailer whose gross vehicle
weight rating (GCWR) exceeds 10,000 pounds,
you’ll have to obtain a Class A CDL. If you don’t
pass a knowledge test on air brakes, your CDL
will include a restriction limiting you to driving
only vehicles without air brakes. - If the low air pressure warning signal is
not working,
it may lead to sudden emergency braking in a
single circuit air system.
The low pressure warning signal should come
on before the air pressure falls below 60 psi. If
it fails to do so, the air pressure may keep
falling without your knowing it. In a single circuit
air system, your spring (emergency) brakes may
come on suddenly. In a dual air system, either
the front or the rear brakes won’t work properly,
the braking distance of your vehicle will
increase, and continued braking will soon cause
the spring brakes to come on. - The air supply pressure gauge shows
how much pressure
is in the air tanks.
The air supply pressure gauge is connected to
the air tank (or tanks) and shows how much air
pressure is present. - You ll know that your brakes are fading if
you have to press the brake pedal harder
than usual to control your speed.
If your brakes begin to fade, you’ll notice that
you have to depress the brake pedal harder to
achieve the same braking effect as before. If
your vehicle is so equipped, check the
application pressure gauge to see just how
much pedal effort you are employing. If the
brakes continue to fade, you may not be able to
slow down or stop at all. - Tractor and straight truck spring brakes
will come fully on when the air pressure
drops to a range of
20 to 45 psi.
An air leak in your brake system can cause your
air brakes to fail. If your air pressure drops to
20-45 psi, your spring brakes will come on
automatically. You can avoid this by pulling over
once the low pressure light comes on so you
can address the problem. - Your vehicle has a dual air brake system.
One of the systems loses air pressure. What
will happen?
Either the front or rear brakes will not be fully
operational.
A dual air brake system has two separate air
brake systems, each with its own air tanks,
hoses, lines, etc. Typically, one system operates
the brakes on the rear axle(s), and the other
system operates the brakes on the front axle. If
one of the systems loses air pressure, either the
front or the rear brakes won’t work properly. As
a result, your vehicle’s braking distance will
increase. If this happens, stop your vehicle and
get the system fixed. Continued braking would
soon cause the spring brakes to come on.
The top 80 most common AIR brakes questions 4 - If your truck or bus has dual parking
control valves, you can use air pressure from
a separate tank to
release the spring emergency/parking brakes.
With a dual parking control valve you can use
the pressure from a separate air tank to give
enough air pressure to release the spring
brakes and move your vehicle. This is very
useful in emergency situations. - The most common type of foundation
brake found on vehicles with air brakes is the
S-cam drum brake.
Foundation brakes are present on each wheel.
In North America as of this writing, S-cam drum
brakes are much more common than wedge or
disc brakes. - If you must make an emergency stop,
brake so that
your vehicle will stay in a straight line.
If you must brake in an emergency, brake in a way
that keeps your vehicle in a straight line while still
letting you turn if necessary. You can use the
“controlled braking” method or (in a vehicle without
antilock brakes) the “stab braking” method. - The braking system that applies the
brakes when the driver presses the brake
pedal is the _ brake system.
service
Air brakes are really three braking systems in one:
the emergency brake, the parking brake, and the
service brake. As these names suggest, when you
depress the brake pedal in normal driving, it’s the
service brake system that applies the brakes. - In a vehicle equipped with air brakes, the
brake pedal
controls the air pressure applied to the
service brakes.
The brake pedal controls the amount of air
pressure applied to the service brakes. The
harder the pedal is pushed, the more air
pressure is applied to the brakes. - To apply the parking brakes under
normal conditions,
let the air out of the brake chambers.
The parking brake control allows you to
evacuate air from the brake chambers. With the
air gone, the spring brakes will come on
automatically and hold your vehicle in place. - Air tanks must be drained
to drain moisture and oil out of the tanks.
Water and oil from the compressor can collect
in the air tanks and must be drained daily to
prevent brake failure. - What are spring brakes?
If the air brakes leak down, springs apply the
brakes to stop the vehicle.
Springs brakes will automatically come on if air
pressure in the tanks drop too low and will not
be able to stop the vehicle. They are used as
emergency brakes and parking brakes. - The use of air brakes on a downgrade is
only a supplement to the
braking effect of the engine.
Going down a hill, you should keep your engine in
a low gear to keep your speed low, and use the
air brakes as a supplemental tool to slow down. - If your truck or bus has dual parking
control valves, you can use air pressure from
a separate tank to
release the spring brakes.
Even after a braking problem has caused the
spring brakes to come on and stop your vehicle,
you may need to move the vehicle again
perhaps to get it out of immediate danger.
Some vehicles such as buses have a separate
air tank that can be used to release the spring
brakes. You can then move the vehicle, after
which you can set the spring brakes again. This
action is controlled by dual parking control
valves: One valve lets air flow from this air tank
to release the spring brakes, and the other
valve sets them again.
The top 80 most common AIR brakes questions 5 - How do you adjust a manual slack adjuster?
Turn the adjusting nut to move the end of the
slack adjuster until the brake shoes contact
the inside of the brake drum. Then turn the
nut the other way to move the brake shoes
away from the brake drum.
To adjust a manual slack adjuster, turn the
adjusting nut so that the S-cam rotates in the
same direction as when the brakes are applied.
Keep turning the nut until the nut stops turning.
At this point, the brake shoes should be
pressing tight against the inside of the brake
drum. Then turn the nut the other way about
one-third of a turn to move the brake shoes
away from the inside of the brake drum. - Every vehicle equipped with air brakes
must have
a supply pressure gauge.
Every vehicle with air brakes has a supply
pressure gauge, which shows how much air
pressure is in the air tanks. Some vehicles also
have an application pressure gauge, which shows
how much air pressure you are applying to the
brakes. Older vehicles also had a manual front
brake limiting valve, which allowed the driver to
cut front brake air pressure in half as a way of
preventing front-wheel skids. However, tests have
shown that full front-wheel braking is safe. - With air brake-equipped vehicles, the
parking brakes should be used
whenever you leave the vehicle unattended.
Because air pressure can leak when you’re
away from your vehicle, it should be set with the
parking brake, to keep it from rolling. There is
an exception to this rule: if the brakes are too
hot or wet, use wheel chocks instead, to avoid
brake damage or freezing. - If oil and water collects in the air tanks,
what could happen to the brakes?
The brakes could fail.
Air tanks must be drained daily to make sure
there is no accumulation of water or oil than can
build up and cause the brakes to fail. - When can you leave your truck
unattended without first applying the
parking brakes or chocking the wheels?
Never.
Never leave your vehicle unattended without
first applying the parking brakes or chocking
the wheels. Your vehicle might roll away and hit
something or someone. - Which of the following can cause brakes
to fail or fade?
Overheating, low air pressure, and not relying
on the engine braking effect
Low air pressure causes your service brakes to
fail, using brakes that have overheated can
cause damage resulting in brake failure, and
having brakes gradually out of alignment can
cause brake fade. - To test the static air leakage rate, you should
turn off the engine, release the parking brake,
and let the system settle.
To test for an air leak, turn off the engine,
release the parking brake (i.e., push the parking
brake control knob in), and see how fast the air
pressure drops. The air pressure should drop
less than 2 psi in one minute for a single vehicle
and less than 3 psi for a combination vehicle. - As part of your vehicle inspection test, if
your vehicle is equipped with air brakes and has
a trailer, you will inspect the air connections
between the truck or tractor and the trailer.
Make sure that the _ are locked in
place and free of damage or air leaks.
glad hands
As part of your vehicle inspection test, if your
vehicle is equipped with air brakes and has a trailer,
you will inspect the air connections between the
truck or tractor and the trailer. These connections
are made with hose couplers (commonly called
glad hands because they vaguely resemble
shaking hands). These glad hands have rubber
seals to form a tight connection and prevent air
from escaping. Make sure that the glad hands are
locked in place and free of damage or air leaks.
The top 80 most common AIR brakes questions 6 - To correct a drive-wheel braking skid,
you should
stop braking, turn quickly, and countersteer.
To recover from a drive-wheel braking skid, first
stop braking. The rear wheels will unlock and
roll again so there will be no more tendency to
slide sideways. Then quickly steer your vehicle
back on course. You may find that your vehicle
will overshoot and keep on turning, possibly
skidding in the opposite direction. Countersteer
to counteract this. Note: This maneuver takes
some practice. You should become proficient in
it before you actually need to use it. - The application pressure gauge shows
the amount of air pressure
that you are applying to the brakes.
If your vehicle is so equipped, the application
pressure gauge will show how much air pressure
you are applying to the brakes. If you have to
apply more pressure than before to achieve the
same braking effect, your brakes are fading. - You are on a steep downgrade and have
reached your safe speed of 40 mph. You
would apply the service brakes until your
speed drops to
35 mph.
When driving down a decline, use your brakes
to keep you near your safe speed. Press the
brakes to slow your vehicle to 5 mph below
your safe speed and then release; repeat as
needed once you surpass your safe speed. - Before you drive a vehicle with a dual air
brake system, wait until the pressure in each
system reaches at least
100 psi.
Before you drive a vehicle with a dual air brake
system, wait until the air compressor has raised
the air pressure to at least 100 psi in both the
primary and secondary systems. Watch the
primary and secondary supply pressure gauges
(or needles, if the system has two needles in
one gauge). - Some air brake systems have an alcohol
evaporator. What may happen if you don t
keep the unit filled with alcohol?
Ice may form in the air storage tanks and
cause the brakes to fail.
Moisture in the warm, compressed air from the air
compressor will condense as the air cools. In
below-freezing temperatures, this moisture can
freeze. Ice in the tanks and air lines can cause the
brakes to stop working. Whenever the air
compressor runs, the alcohol evaporator puts
alcohol vapor into the air brake system to help
prevent water in the lines or valves from freezing.
(Alcohol has a lower freezing point than water.) - How should you check that the spring
brakes come on when the air pressure in the
system drops below a certain level?
Park on level ground, chock the wheels,
release the parking brake when you have
enough air pressure, shut the engine off, and
repeatedly press and release the brake pedal.
When doing your final check of your air brake,
you should cause your vehicle to have low air
pressure, to be sure the warning signal comes
on, by turning the engine off, with the wheels
chocked, and pushing on and off the brakes to
release the air pressure. Once you re sure the
low pressure warning is viable, continue doing
the same until the spring brakes come on
automatically, usually between 20- 45 psi. - To make an emergency stop with the
stab braking method, you should
depress the brake pedal as hard as you can,
release the brakes when the wheels lock, and
apply the brakes again when the wheels start
to roll.
To stop with stab braking, brake hard, release
the brakes once the wheels lock, and then
brake hard once the wheels start to roll again.
(Clearly, this method won t work if your vehicle
has antilock brakes, because the wheels won t
lock. Use controlled braking instead.)
The top 80 most common AIR brakes questions 7 - If the air compressor should develop a
leak, what will keep the air in the tanks?
The one-way check valve
Federal regulations require a one-way check
valve or an equivalent device to be located at
the entrance to the first air tank. (In a dual air
brake system, there must be a one-way check
valve at the entrance to both the primary and
secondary air tanks.) If the air compressor or
the line connecting it to the air tank develops a
leak, the one-way check valve will prevent the
air from going out. - The braking power of the spring brakes
depends on whether the service brakes are in
proper adjustment.
The braking power of the spring brakes
depends on whether the service brakes are in
proper adjustment. If they are not, neither the
service brakes nor the emergency/parking
brakes will work properly. - Excessive heat caused by overuse of the
service brakes can cause
the brakes to fade.
Excessive heat caused by overuse of the
brakes can expand the brake drums away from
the brake pads or shoes and affect the brake
lining, reducing friction. The brakes will fade
(lose their effectiveness). On a long or steep
downgrade, rely on engine braking more than
the service brakes. - When you pull hard on a slack adjuster, if
it has more than _ of free play, the
slack adjuster may need adjustment.
1 inch
During the walk-around inspection, pull hard on
each slack adjuster that you can reach. If it moves
more than an inch, it probably needs adjustment. - Air brakes use _ to make the
brakes work.
compressed air
Air brakes use compressed air to make the
brakes work. - If the spring brakes are on, when should
you push the brake pedal?
Never.
Applying the spring brakes and the air brakes at
the same time may be too much force on the
brakes and can actually damage them. - What is the first thing you should do if
the low air pressure warning comes on?
Stop.
If the low air pressure warning comes on, there
may be a leak in the system. Pull over and park
your vehicle as soon as possible. If the air
pressure drops too low, the spring brakes will
come on automatically, which will stop your
vehicle but not necessarily in a safe way.
Sudden application of the spring brakes while
you re driving might result in a skid. - During your final air brake check, if the
air pressure does not build up fast enough,
the air pressure may drop too low during driving.
The manufacturer s specifications should state
how fast the air pressure should build up once
the engine has reached normal operating
speed. If the air pressure does not build up fast
enough during your final air brake check, the air
pressure may not replenish quickly enough
during normal braking either. Don t drive the
vehicle until the cause has been found and
corrected. - The S-cam
forces the brake shoes against the inside of
the brake drum.
When the S-cam rotates, it forces the brake
shoes apart and against the inside of the brake
drum. Friction will slow the rotation of the wheel. - The parking brake control
lets the air out of the brake chambers.
When you apply the parking brake control, air is
released from the brake chambers, causing the
springs to apply the brakes and hold your
vehicle in place.
The top 80 most common AIR brakes questions 8 - Under ideal conditions, the average
driver of a truck or bus equipped with air
brakes and traveling at 55 mph would
require what stopping distance?
More than 400 feet
The stopping distance equals the sum of the
perception distance, reaction distance, brake
lag distance, and braking distance. Under ideal
conditions, the average driver of a bus or large
truck equipped with air brakes and traveling at
55 mph would require about 451 feet to come
to a complete stop. The length of a football field
including the two end zones is 360 feet. - With S-cam drum brakes, when the brake
chamber fills with air, air pressure pushes
the push rod out, moving the
and thus rotating the .
slack adjuster; brake camshaft
To apply the S-cam drum brakes, air is let into
each brake chamber. Air pressure pushes the
push rod out, moving the slack adjuster, thus
rotating the brake camshaft. As the S-cam on
the camshaft rotates, it forces the brake shoes
against the inside of the brake drum. - What is the best way to test your
vehicle’s low air pressure warning signal?
With the engine off, step on and off the brake
pedal to lower the air pressure below 60 psi.
The best way to test the low air pressure warning
signal is to release the parking brake (all
vehicles), release the tractor protection valve (on
combination vehicles), and turn off the engine of
your vehicle while leaving the electrical power
on. Then step on and off the brake pedal. The
low air pressure warning signal should come on
when the pressure falls below 60 psi. You should
never need to get down to 30 psi. - To test the air service brakes,
brake firmly while slowly moving forward.
To test the service brakes, wait for normal air
pressure. Then move your vehicle forward
slowly (at about 5 mph) and brake hard. Check
whether your vehicle stops promptly without
pulling to one side or the other. - What is the purpose of an alcohol evaporator?
To prevent a buildup of ice in the air brake system.
Alcohol evaporators put alcohol vapor into the
air system to help prevent water in the lines or
valves from freezing. Ice in the lines or valves
can cause them to stop working. - S-cam drum brakes have an S-cam in
each brake. Why is it called an S-cam?
It is shaped like the letter S .
The S-cam really is S-shaped. As it rotates, it
forces the brake shoes away from each other
and toward the inside of the brake drum. - A converter dolly with antilock brakes
(ABS) is required to have
a yellow lamp on the left side.
A converter dolly with antilock brakes is
required to have a yellow ABS lamp on its left
side. All converter dollies built since March 1,
1998 have antilock brakes. - Which of the following is the most
important thing about hard braking?
Don t lock the wheels for longer than an instant.
When braking hard, don t lock the wheels for
longer than an instant (which is done in stab
braking). If the wheels stay locked, they will lose
the grip of the road and your vehicle may skid
or even jackknife. - In controlled braking, you
brake in a straight line.
In controlled braking, you brake as hard as you
can without locking the wheels. As you do this,
keep your vehicle going relatively straight. - The air loss rate for a straight truck or
bus with the engine off and the brakes on
should not be more than
3 psi in one minute.
When testing for an air leak, you should turn off
the engine and hold the brake for one minute:
the air pressure should not drop more than 3
psi in that one minute.
The top 80 most common AIR brakes questions 9
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