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 rake 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.
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.
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.)
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.
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.
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