CDL Air Brakes Practice Test

All 30 questions with answers

For reading through. Tap "Show answer" under a question to see the answer and the reason.

1. Air brakes are really three braking systems. Which three?

   1. Engine, exhaust and wheel
   2. Service, parking and emergency
   3. Primary, secondary and tertiary
   4. Front, rear and trailer
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   B. Service, parking and emergency The service brake is used in normal driving, the parking brake holds the parked vehicle, and the emergency system uses parts of both to stop the vehicle if the service system fails. (CDL manual)

2. The air compressor governor usually stops the compressor pumping (cut-out) at about:

   1. 125 psi
   2. 175 psi
   3. 100 psi
   4. 150 psi
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   A. 125 psi Cut-out is around 125 psi; the compressor stops pumping air into the tanks. (CDL manual)

3. The governor lets the compressor start pumping again (cut-in) at about:

   1. 80 psi
   2. 100 psi
   3. 60 psi
   4. 125 psi
   Show answer

   B. 100 psi When tank pressure falls to about 100 psi the governor lets the compressor pump again. (CDL manual)

4. The safety relief valve on the first tank usually opens at:

   1. 125 psi
   2. 200 psi
   3. 150 psi
   4. 100 psi
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   C. 150 psi It protects the tank and system from too much pressure, usually opening at 150 psi. If it releases air, something is wrong: have it fixed. (CDL manual)

5. Why must air tanks be drained?

   1. To lower the pressure before driving
   2. To test the compressor
   3. To cool the brakes
   4. Water and compressor oil collect in them and can make the brakes fail
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   D. Water and compressor oil collect in them and can make the brakes fail Compressed air carries water and some compressor oil into the tanks. In cold weather the water can freeze and cause brake failure. (CDL manual)

6. Tanks with manual drain valves should be drained:

   1. At the end of each day of driving
   2. Once a month
   3. Only when the low-air warning comes on
   4. Only in winter
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   A. At the end of each day of driving Drain them completely at the end of each day. Automatic drains still need checking. (CDL manual)

7. What does an alcohol evaporator do?

   1. Puts alcohol into the air system to reduce the risk of ice
   2. Cleans oil out of the tanks
   3. Tests the driver for alcohol
   4. Cools the compressor
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   A. Puts alcohol into the air system to reduce the risk of ice Alcohol in the air lowers the freezing point of water in the system. Check and fill the alcohol container every day in cold weather. (CDL manual)

8. According to the CDL manual, the low air pressure warning must come on before tank pressure drops below:

   1. 60 psi
   2. 90 psi
   3. 45 psi
   4. 20 psi
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   A. 60 psi The manual says before it drops below 60 psi (half of that in some older vehicles). The federal equipment rule is written as 55 psi or half the governor cut-out pressure, whichever is less. (CDL manual; 49 CFR 393.51)

9. In most vehicles, the spring brakes come on automatically when air pressure falls to:

   1. 5 to 10 psi
   2. 60 to 80 psi
   3. 90 to 100 psi
   4. 20 to 45 psi
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   D. 20 to 45 psi Somewhere in the 20 to 45 psi range, depending on the vehicle. The low-air warning should always come on first. (CDL manual)

10. Build-up test on a dual air system: pressure should rise from 85 to 100 psi within:

    1. 2 minutes
    2. 15 seconds
    3. 3 minutes
    4. 45 seconds
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    D. 45 seconds With the engine at operating rpm, 85 to 100 psi within 45 seconds in dual air systems. Older single systems: 50 to 90 psi within 3 minutes. (CDL manual)

11. Leakage test, engine off, brakes released: a single vehicle should lose less than:

    1. 10 psi in one minute
    2. 1 psi in one minute
    3. 2 psi in one minute
    4. 4 psi in one minute
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    C. 2 psi in one minute Less than 2 psi a minute for a single vehicle, less than 3 psi for a combination. (CDL manual)

12. Leakage test with the brakes fully applied: after the first drop, a combination vehicle should lose less than:

    1. 4 psi in one minute
    2. 2 psi in one minute
    3. 3 psi in one minute
    4. 8 psi in one minute
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    A. 4 psi in one minute With a firm brake application: less than 3 psi a minute for a single vehicle and 4 psi for a combination. More means a leak to fix before driving. (CDL manual)

13. Why do air brakes take longer to start working than hydraulic brakes?

    1. The drums need to warm up
    2. The governor has to cut in first
    3. Air takes time to flow through the lines to the brakes
    4. The pedal is harder to push
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    C. Air takes time to flow through the lines to the brakes Brake lag: about half a second for the air to reach the brakes after you press the pedal. (CDL manual)

14. At 55 mph on dry pavement, brake lag adds about how much to total stopping distance?

    1. 300 feet
    2. 5 feet
    3. 100 feet
    4. 32 feet
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    D. 32 feet About 32 feet on top of the perception, reaction and braking distance a hydraulic-brake vehicle would need. (CDL manual)

15. With the parking brakes released, you pull hard on a slack adjuster by hand. It should not move more than about:

    1. 1 inch
    2. It should move freely
    3. 1/8 inch
    4. 3 inches
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    A. 1 inch If a pushrod-type slack adjuster moves more than about an inch, the brake probably needs adjusting. Out-of-adjustment brakes are a very common problem found at inspections. (CDL manual)

16. In S-cam brakes, what forces the brake shoes against the drum?

    1. A spring on each shoe
    2. Hydraulic fluid
    3. The S-shaped cam, turned by the brake camshaft
    4. The wheel's rotation
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    C. The S-shaped cam, turned by the brake camshaft Air pushes the pushrod, which turns the slack adjuster and camshaft. The S-cam twists and spreads the shoes against the inside of the drum. (CDL manual)

17. In a typical dual air brake system, the primary system usually works the:

    1. Rear axle or axles
    2. Parking brake only
    3. Trailer only
    4. Front axle only
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    A. Rear axle or axles The primary system usually works the rear axles and the secondary system the front axle (and sometimes one rear axle). Each has its own tanks and lines. (CDL manual)

18. Before driving a vehicle with a dual air system, wait until:

    1. The safety valve opens
    2. The low-air warning comes on
    3. Pressure reaches 60 psi in either system
    4. Pressure reaches at least 100 psi in both systems
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    D. Pressure reaches at least 100 psi in both systems Allow the compressor to build at least 100 psi in both the primary and secondary systems, and watch the warning light go off. (CDL manual)

19. The low air pressure warning comes on while you are driving. You should:

    1. Pull the parking brake immediately at speed
    2. Keep going to the next shop
    3. Stop and safely park as soon as possible
    4. Pump the brake pedal to build pressure
    Show answer

    C. Stop and safely park as soon as possible Low air may mean a leak or failure, and the spring brakes could come on without warning. Stop somewhere safe while you still control the stop. (CDL manual)

20. When should you not set the parking brakes?

    1. At night
    2. When the trailer is loaded
    3. When the brakes are very hot or wet in freezing temperatures
    4. On a slight grade
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    C. When the brakes are very hot or wet in freezing temperatures Very hot brakes can be damaged, and wet brakes can freeze on. Use wheel chocks instead, and let hot brakes cool first. (CDL manual)

21. Is it true that you should limit the front brakes on a slippery road?

    1. Only with an empty trailer
    2. Only on ice
    3. Yes, front brakes cause skids
    4. No, front brakes are good under all conditions
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    D. No, front brakes are good under all conditions Tests have shown front-wheel skids from braking are unlikely even on ice. Many newer vehicles have no front brake limiting valve at all. (CDL manual)

22. What does the supply pressure gauge show?

    1. How much pressure is in the air tanks
    2. How hard you are pressing the brake pedal
    3. Tire pressure
    4. Oil pressure in the compressor
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    A. How much pressure is in the air tanks Vehicles with air brakes have a gauge showing tank pressure; dual systems show each half. (CDL manual)

23. On a downgrade you need more and more application pressure to hold the same speed. This means:

    1. The gauge is broken
    2. The compressor is failing
    3. The brakes are fading; slow down and use a lower gear
    4. The tanks need draining
    Show answer

    C. The brakes are fading; slow down and use a lower gear The application pressure gauge shows how much air goes to the brakes. Needing more for the same slowing means the brakes are fading. (CDL manual)

24. A "wig wag" low-pressure warning drops into the driver's view when pressure falls below about:

    1. 80 psi
    2. 60 psi
    3. 20 psi
    4. 100 psi
    Show answer

    B. 60 psi It drops a mechanical arm into view below about 60 psi. Some types rise out of view when pressure goes back above 60; others must be reset by hand. (CDL manual)

25. In newer vehicles, the parking brake control is a:

    1. Blue lever on the floor
    2. Red eight-sided knob; push to set
    3. Foot pedal to the left of the clutch
    4. Yellow diamond-shaped knob; pull out to set, push in to release
    Show answer

    D. Yellow diamond-shaped knob; pull out to set, push in to release Yellow, diamond-shaped, push-pull. The red eight-sided knob is the trailer air supply on a tractor. (CDL manual)

26. How do you test that the low-air warning and spring brakes work?

    1. Disconnect the compressor belt
    2. Drain one tank
    3. With the engine off, step on and off the brake pedal to reduce tank pressure
    4. Drive and pull the parking brake
    Show answer

    C. With the engine off, step on and off the brake pedal to reduce tank pressure Fan off the air by repeatedly applying and releasing the brake. The warning should come on before about 60 psi, and the parking brake knob should pop out between 20 and 45 psi on most vehicles. (CDL manual)

27. The emergency braking system:

    1. Uses parts of the service and parking brake systems
    2. Works only on trailers
    3. Is a separate set of brakes on the front axle
    4. Is the engine brake
    Show answer

    A. Uses parts of the service and parking brake systems It is not a fourth set of brakes; it uses parts of the other two to stop the vehicle when the service system fails. (CDL manual)

28. A brake drum has a crack longer than half the width of the friction area. It is:

    1. Unsafe: it must not be used
    2. Acceptable on the trailer
    3. Only a problem in cold weather
    4. Fine if the brakes still work
    Show answer

    A. Unsafe: it must not be used Drums and discs must not have cracks longer than one half of the width of the friction area. (CDL manual)

29. The air compressor is driven by:

    1. The transmission output shaft only
    2. The wheels
    3. A separate electric motor
    4. The engine, through gears or a belt
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    D. The engine, through gears or a belt It is connected to the engine by gears or a V-belt, and may be cooled by air or engine coolant. (CDL manual)

30. The brake pedal controls the air brakes by:

    1. Directly pulling the brake shoes
    2. Pushing fluid to the wheels
    3. Opening a valve that lets more or less air to the brakes
    4. Turning the compressor on
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    C. Opening a valve that lets more or less air to the brakes The foot valve lets air go to the brakes: the harder you press, the more pressure is applied. Releasing lets air out, which is why fanning the brakes uses up tank pressure. (CDL manual)