CDL Air Brakes Practice Test

25 practice questions

Choose an answer, then check it under Show answer.

1. Question 1 of 25Medium

   When using the stab braking technique, how long can it take for the wheels to start rolling again after releasing the brake pedal?

   1. AA. Up to seven seconds
   2. BB. Up to one second
   3. CC. Up to three seconds
   4. DD. Up to five seconds
   Show answer to question 1

   Answer: B. Up to one second

   After releasing the pedal in stab braking, it can take up to one second for the locked wheels to begin rolling again. Drivers commonly mistake this with three seconds, which is the recommended duration of a downgrade brake application.

2. Question 2 of 25Hard

   Why may automatic air tanks be equipped with electric heating devices?

   1. AA. To help prevent the automatic drain from freezing in cold weather
   2. BB. To warm the compressed air before it reaches the brake chambers
   3. CC. To keep tank pressure from falling below the cut-in level
   4. DD. To help the alcohol evaporator work at low temperatures
   Show answer to question 2

   Answer: A. To help prevent the automatic drain from freezing in cold weather

   The manual notes that automatic air tanks are available with electric heating devices that help prevent freezing of the automatic drain in cold weather. The heater is there for the drain, not for warming air on its way to the brake chambers, and it does not control tank pressure or assist the alcohol evaporator.

3. Question 3 of 25Medium

   On a tractor-trailer, air pressure is fanned off until it reaches the manufacturer's specification. What should happen at that point?

   1. AA. The tractor protection valve and parking brake valve should close (pop out)
   2. BB. The air compressor should start and build pressure to 85 psi
   3. CC. The service brake pedal should become firm and stop moving
   4. DD. The low pressure warning signal should turn off and reset
   Show answer to question 3

   Answer: A. The tractor protection valve and parking brake valve should close (pop out)

   When pressure falls to the manufacturer's specification, the tractor protection valve and parking brake valve should close, or pop out. This causes the spring brakes to come on.

4. Question 4 of 25Hard

   While driving, the primary system gauge drops to a very low reading. Which brakes will not be operating fully?

   1. AA. The parking brakes on all axles
   2. BB. The rear axle regular brakes
   3. CC. The front axle regular brakes
   4. DD. The trailer brakes only
   Show answer to question 4

   Answer: B. The rear axle regular brakes

   The manual explains that if one system is very low on pressure, either the front or the rear brakes will not be operating fully, and the primary system is the one that operates the regular brakes on the rear axle or axles. Because only one system has lost pressure, the vehicle will still brake, but it will take longer to stop, so you should park safely and have the system fixed.

5. Question 5 of 25Easy

   When driving down a steep grade, each brake application should reduce your vehicle speed to approximately what level before you release the brakes?

   1. AA. 15 mph below your safe speed
   2. BB. 20 mph below your safe speed
   3. CC. 10 mph below your safe speed
   4. DD. 5 mph below your safe speed
   Show answer to question 5

   Answer: D. 5 mph below your safe speed

   Proper downgrade braking requires reducing speed to approximately 5 mph below your designated safe speed before releasing the brakes. Reducing speed by 10 mph below the safe speed is an overly long brake application that can cause unnecessary brake heating.

6. Question 6 of 25Easy

   In an s-cam brake, what makes the slack adjuster move when you push the brake pedal?

   1. AA. Air pressure pushing the brake chamber rod out
   2. BB. A wedge driven between the shoe ends
   3. CC. Hydraulic fluid coming from the foot valve
   4. DD. A spring pulling the camshaft back
   Show answer to question 6

   Answer: A. Air pressure pushing the brake chamber rod out

   Pushing the pedal lets air into each brake chamber, and the air pressure pushes the rod out, moving the slack adjuster and twisting the brake camshaft. A wedge driven between shoe ends is how a wedge brake works, not an s-cam brake.

7. Question 7 of 25Medium

   While checking brake drums, what is the longest crack a drum may have?

   1. AA. One half inch, no matter how large the drum is
   2. BB. One half the width of the friction area
   3. CC. One inch, no matter how large the drum is
   4. DD. The full width of the friction area
   Show answer to question 7

   Answer: B. One half the width of the friction area

   The CDL manual states that brake drums must not have cracks longer than one half the width of the friction area. A crack running the full width of the friction area is beyond that limit and is not acceptable.

8. Question 8 of 25Medium

   A dual air system gauge has two needles in one dial. What do the two needles show?

   1. AA. Compressor output at each axle
   2. BB. Pressure in the tractor and the trailer
   3. CC. Pressure in the primary and secondary systems
   4. DD. Air temperature in each system
   Show answer to question 8

   Answer: C. Pressure in the primary and secondary systems

   The manual tells you to watch the primary and secondary gauges, or the needles if the system has two needles in one gauge, so each needle shows the pressure in one of the two systems. The needles do not show air temperature or compressor output.

9. Question 9 of 25Medium

   While you are driving, what holds the powerful spring brakes back?

   1. AA. An electric latch in the parking brake knob
   2. BB. Air pressure in the system
   3. CC. Hydraulic pressure from the master cylinder
   4. DD. Mechanical force from the springs
   Show answer to question 9

   Answer: B. Air pressure in the system

   While driving, air pressure holds the springs back; when the air pressure is removed, the springs put the brakes on. Mechanical force is tempting because it is what actually applies the brakes, but it is air pressure that keeps them released while you drive.

10. Question 10 of 25Easy

    Which factor must be added to perception distance, reaction distance, and braking distance to calculate total stopping distance for air brakes?

    1. AA. Wheel lockup distance
    2. BB. Compressor recovery distance
    3. CC. Clutch engagement distance
    4. DD. Brake lag distance
    Show answer to question 10

    Answer: D. Brake lag distance

    Air brake systems introduce brake lag, which is the time required for air to flow through the lines to the brakes, making brake lag distance one of the four components of total stopping distance. Compressor recovery relates to tank air pressure replenishment rather than stopping distance.

11. Question 11 of 25Easy

    When performing the service brake test, how fast should the vehicle be moving forward before you apply the brakes firmly?

    1. AA. About 5 mph
    2. BB. About 35 mph
    3. CC. About 25 mph
    4. DD. About 15 mph
    Show answer to question 11

    Answer: A. About 5 mph

    To test the service brakes, you release the parking brake, move the vehicle forward slowly at about 5 mph, and press the brake pedal firmly. Higher speeds like 15 or 25 mph are dangerous for a test because brake pulling or delayed stopping could cause a loss of vehicle control.

12. Question 12 of 25Easy

    If one air system in a dual air brake vehicle loses pressure while you are driving, what effect will this have on braking?

    1. AA. Braking distance will remain unchanged because the secondary circuit compensates.
    2. BB. It will take longer to bring the vehicle to a stop.
    3. CC. The vehicle will stop in a shorter distance because the remaining brakes grab harder.
    4. DD. Braking power will shift entirely to the trailer brakes.
    Show answer to question 12

    Answer: B. It will take longer to bring the vehicle to a stop.

    When one system is very low on pressure, either the front or rear brakes will not operate fully, which means it will take longer to stop. The remaining brakes cannot shorten stopping distance or maintain standard stopping performance.

13. Question 13 of 25Easy

    In a drum foundation brake, where is the braking mechanism located?

    1. AA. Inside the slack adjuster
    2. BB. Inside the brake drum
    3. CC. Inside the air compressor
    4. DD. Inside the caliper
    Show answer to question 13

    Answer: B. Inside the brake drum

    The CDL manual says brake drums are mounted on each end of the axles and the braking mechanism is inside the drum. The brake shoes and linings are pushed against the inside of the drum to create friction that slows the vehicle. A caliper is part of a disc brake, not a drum brake.

14. Question 14 of 25Hard

    Why does a heavily loaded vehicle take a long distance to stop if the spring brakes deploy automatically?

    1. AA. The spring brakes do not work on all axles.
    2. BB. The drums will expand too far for the springs to make contact.
    3. CC. The spring brakes apply only to the steering axle.
    4. DD. The heavy load prevents the springs from expanding fully.
    Show answer to question 14

    Answer: A. The spring brakes do not work on all axles.

    When air pressure drops into the 20 to 45 psi range, spring brakes apply automatically, but they do not act on all axles. Because not all wheels receive braking force, stopping a heavily loaded vehicle with only the spring brakes requires a much longer distance.

15. Question 15 of 25Hard

    In an emergency, a driver manually adjusts an automatic slack adjuster to bring the push rod stroke within limits. What is likely to happen?

    1. AA. The brake will probably soon be back out of adjustment
    2. BB. The brake linings will wear more evenly
    3. CC. The adjuster will be permanently repaired
    4. DD. The air compressor governor will reset itself
    Show answer to question 15

    Answer: A. The brake will probably soon be back out of adjustment

    Manual adjustment of an automatic adjuster is only a temporary measure because it usually does not fix the underlying adjustment problem, so the brake will likely soon be out of adjustment again. It does not repair the adjuster; in fact, routine adjustment may cause premature wear.

16. Question 16 of 25Easy

    Each of the two systems in a dual air brake system has its own:

    1. AA. Warning light and buzzer
    2. BB. Compressor and governor
    3. CC. Brake pedal and controls
    4. DD. Air tanks, hoses and lines
    Show answer to question 16

    Answer: D. Air tanks, hoses and lines

    The manual describes a dual air system as two separate systems, each with its own air tanks, hoses, lines and so on. The two systems share a single set of brake controls, so a separate brake pedal for each system is not correct.

17. Question 17 of 25Medium

    A mechanical arm suddenly drops down into the driver's view while operating an air brake vehicle equipped with a wig wag device. What warning does this device give?

    1. AA. The manual front brake limiting valve has engaged
    2. BB. The compressor governor cutout setting has failed
    3. CC. Brake application pressure has exceeded 55 psi
    4. DD. System air pressure has dropped below 55 psi
    Show answer to question 17

    Answer: D. System air pressure has dropped below 55 psi

    A wig wag is a low air pressure warning device that drops a mechanical arm into the driver's view when pressure in the system drops below 55 psi. It signals low reservoir pressure, not the amount of brake application pressure being used.

18. Question 18 of 25Medium

    What is the concern with manually adjusting an automatic slack adjuster to bring the pushrod stroke within legal limits?

    1. AA. It raises the governor cut-out pressure above 125 psi
    2. BB. It is the recommended routine maintenance before each trip
    3. CC. It masks a mechanical problem and can cause premature adjuster wear
    4. DD. It permanently corrects the underlying brake defect
    Show answer to question 18

    Answer: C. It masks a mechanical problem and can cause premature adjuster wear

    The CDL manual warns that manual adjustment generally masks a mechanical problem, does not fix it, and routine adjustment likely causes premature wear of the adjuster. It is not a permanent repair and is not routine maintenance.

19. Question 19 of 25Medium

    Why should you avoid using the parking brakes right after a steep downgrade when the brakes are very hot?

    1. AA. The parking brakes will not hold on a grade.
    2. BB. The brakes will release automatically when hot.
    3. CC. The air pressure will drop too quickly.
    4. DD. The heat can damage the parking brakes.
    Show answer to question 19

    Answer: D. The heat can damage the parking brakes.

    If parking brakes are used while they are very hot, they can be damaged by the heat. Let hot brakes cool first and use wheel chocks on a level surface to hold the vehicle.

20. Question 20 of 25Medium

    If pressure drops dangerously low in one part of a dual air brake system, why does the vehicle take longer to stop?

    1. AA. The single set of brake controls stops sending mechanical input
    2. BB. The engine automatically cuts fuel delivery to slow the vehicle
    3. CC. The air compressor stops pumping air into the remaining lines
    4. DD. Either the front or rear brakes will not be operating fully
    Show answer to question 20

    Answer: D. Either the front or rear brakes will not be operating fully

    In a dual air brake system, one circuit operates the rear brakes and the other operates the front brakes. If one system loses pressure, either the front or the rear brakes will not be operating fully, which directly increases stopping distance. The controls do not stop working entirely, but braking force is significantly reduced.

21. Question 21 of 25Medium

    What does ABS do to your normal braking capability?

    1. AA. It increases normal braking capability
    2. BB. It replaces normal brakes at low speed
    3. CC. It does not decrease or increase it
    4. DD. It decreases normal braking capability
    Show answer to question 21

    Answer: C. It does not decrease or increase it

    ABS is an addition to normal brakes and does not decrease or increase normal braking capability; it only activates when wheels are about to lock. The option saying ABS increases braking capability wrongly suggests it makes the normal brakes stronger.

22. Question 22 of 25Medium

    A tractor-trailer has ABS on only one axle. What does the CDL manual say this does for the driver?

    1. AA. It prevents any trailer from swinging out
    2. BB. It requires the driver to pump the brake pedal
    3. CC. It still gives more control over the vehicle during braking
    4. DD. It gives no benefit unless every axle has ABS
    Show answer to question 22

    Answer: C. It still gives more control over the vehicle during braking

    Having ABS on only the tractor, only the trailer, or even only one axle still gives more control during braking. The manual does not say it must be on every axle to help, and it still tells the driver to brake normally.

23. Question 23 of 25Hard

    When testing the air compressor governor, at what approximate pressures should the compressor cut in and cut out?

    1. AA. Cut in at about 80 psi and cut out at about 105 psi
    2. BB. Cut in at about 100 psi and cut out at about 125 psi
    3. CC. Cut in at about 60 psi and cut out at about 90 psi
    4. DD. Cut in at about 115 psi and cut out at about 140 psi
    Show answer to question 23

    Answer: B. Cut in at about 100 psi and cut out at about 125 psi

    Pumping by the air compressor should begin at roughly 100 psi (cut-in) and stop at about 125 psi (cut-out), depending on manufacturer specifications. Pressures around 60 psi to 90 psi are too low, as the low air warning generally activates around 60 psi. A governor that does not cut in and cut out at the proper levels may fail to maintain sufficient air pressure for safe braking.

24. Question 24 of 25Hard

    In a dual air brake system, the secondary system operates the regular brakes on the front axle and may also operate regular brakes on which axle?

    1. AA. All drive axles
    2. BB. The steer axle exclusively
    3. CC. One rear axle
    4. DD. All trailer axles
    Show answer to question 24

    Answer: C. One rear axle

    The secondary system typically operates the regular brakes on the front axle and possibly one rear axle, while the primary system handles the rear axles. It does not operate all drive axles or trailer axles exclusively.

25. Question 25 of 25Hard

    In stab braking, what happens if you re-apply the brakes before the wheels start rolling?

    1. AA. The air pressure will build faster
    2. BB. The ABS will release the brakes
    3. CC. The vehicle won't straighten out
    4. DD. The trailer brakes will lock first
    Show answer to question 25

    Answer: C. The vehicle won't straighten out

    The manual warns that if you re-apply the brakes before the wheels start rolling, the vehicle won't straighten out. You should wait until the wheels start rolling, which can take up to one second after releasing the brakes.

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