CDL Air Brakes Practice Test 2026: Practice test 2

1. Question 1 of 25

   What is a wig wag?

   • AA type of flashing brake light on the trailer
   • BA device that wags the trailer to warn drivers
   • CA switch that releases the spring brakes
   • DA metal arm that drops into view when air is low
   Show answer

   Answer: D. A metal arm that drops into view when air is low

   A wig wag is a low pressure warning device: a mechanical arm that drops into your view when pressure falls below 60 psi.

2. Question 2 of 25

   What does the stop light switch do in an air brake system?

   • ATurns on the brake lights when you brake
   • BApplies the spring brakes when air is low
   • CStops the compressor at cut-out pressure
   • DTurns off the engine when pressure is low
   Show answer

   Answer: A. Turns on the brake lights when you brake

   An air-pressure switch turns on the brake lights when the brakes are applied.

3. Question 3 of 25

   Some older vehicles have a front brake limiting valve with a "normal" and "slippery" setting. Where should you keep it?

   • AIt does not matter
   • BIn the "normal" position
   • CIn the "slippery" position at all times
   • DIn "slippery" on dry roads only
   Show answer

   Answer: B. In the "normal" position

   The slippery setting cuts air to the front brakes by half, which reduces stopping power. Keep it in normal.

4. Question 4 of 25

   How are spring brakes held off during normal driving?

   • ABy air pressure
   • BBy the parking brake cable
   • CBy the governor
   • DBy hydraulic fluid
   Show answer

   Answer: A. By air pressure

   Spring brakes are held back by air pressure. When air is lost, the springs put on the brakes.

5. Question 5 of 25

   When will the spring brakes on a tractor or straight truck come fully on?

   • AWhen air pressure falls below 100 psi
   • BWhen air pressure falls to about 20 to 45 psi
   • CWhen air pressure rises above 125 psi
   • DOnly when the driver pulls the yellow knob
   Show answer

   Answer: B. When air pressure falls to about 20 to 45 psi

   Spring brakes come fully on when air pressure drops to a range of 20 to 45 psi, typically 20 to 30 psi.

6. Question 6 of 25

   The low air pressure warning comes on while you are driving. What should you do?

   • APump the brakes to build pressure
   • BSpeed up to charge the tanks faster
   • CStop and park safely as soon as possible
   • DKeep driving to the next terminal
   Show answer

   Answer: C. Stop and park safely as soon as possible

   There may be an air leak. Controlled braking is only possible while enough air remains, so stop safely right away.

7. Question 7 of 25

   Why should you not push the brake pedal down when the spring brakes are on?

   • AIt turns off the ABS
   • BThe combined force can damage the brakes
   • CIt will raise the air pressure
   • DIt will release the spring brakes
   Show answer

   Answer: B. The combined force can damage the brakes

   Pushing the pedal while the spring brakes are on adds the two forces together and can damage the brakes.

8. Question 8 of 25

   What does the parking brake control look like on newer vehicles?

   • AA green lever beside the seat
   • BA red, eight-sided knob
   • CA blue, round push-pull knob
   • DA yellow, diamond-shaped knob
   Show answer

   Answer: D. A yellow, diamond-shaped knob

   Newer vehicles use a yellow, diamond-shaped knob. The red eight-sided knob is the trailer air supply.

9. Question 9 of 25

   How do you use the yellow parking brake knob?

   • APush it in to apply; pull it out to release
   • BPull it out to apply; push it in to release
   • CHold it down for as long as you are parked
   • DTurn it to the left to apply the brakes
   Show answer

   Answer: B. Pull it out to apply; push it in to release

   Pull the knob out to put the parking brakes on, and push it in to release them.

10. Question 10 of 25

    What does a modulating control valve let you do?

    • ADrain all the air tanks from inside the cab
    • BApply the spring brakes a little at a time
    • CChange the governor's cut-in and cut-out settings
    • DTurn the antilock brake system on and off
    Show answer

    Answer: B. Apply the spring brakes a little at a time

    A modulating control valve is a lever on the dash that lets you apply the spring brakes a little at a time.

11. Question 11 of 25

    Some vehicles have dual parking control valves with a separate small air tank. What is it for?

    • AFilling the tires with air when one goes low
    • BGiving the air horn its own supply of air
    • CReleasing the spring brakes briefly in an emergency
    • DMaking the parking brakes hold more strongly
    Show answer

    Answer: C. Releasing the spring brakes briefly in an emergency

    The separate tank holds only enough air to release the spring brakes a few times, so plan your moves carefully.

12. Question 12 of 25

    Truck tractors with air brakes built on or after what date must have antilock brakes (ABS)?

    • AJuly 1, 2010
    • BJanuary 1, 1990
    • CMarch 1, 2005
    • DMarch 1, 1997
    Show answer

    Answer: D. March 1, 1997

    Air-braked truck tractors built on or after March 1, 1997 must have ABS. Other air-braked vehicles built on or after March 1, 1998 must too.

13. Question 13 of 25

    Which air-braked vehicles built on or after March 1, 1998 must have ABS?

    • ATrucks, buses, trailers and converter dollies
    • BOnly school buses and transit buses
    • COnly tank vehicles hauling hazmat
    • DOnly truck tractors pulling trailers
    Show answer

    Answer: A. Trucks, buses, trailers and converter dollies

    Besides tractors (from March 1, 1997), other air brake vehicles built on or after March 1, 1998 must have antilock brakes.

14. Question 14 of 25

    What does it mean if your ABS lamp comes on for a moment when you start the vehicle and then goes out?

    • AThe ABS has failed
    • BIt is a normal bulb check
    • CThe parking brake is off
    • DThe air tanks are empty
    Show answer

    Answer: B. It is a normal bulb check

    Newer systems light the ABS lamp briefly at start-up as a bulb check, then turn it off.

15. Question 15 of 25

    Does ABS shorten your stopping distance?

    • AYes, it always cuts stopping distance in half
    • BYes, but only on dry roads
    • CNo, it makes stopping distance much longer
    • DNot necessarily; it helps you keep steering control
    Show answer

    Answer: D. Not necessarily; it helps you keep steering control

    ABS does not change normal braking ability or always shorten stopping distance. It prevents lockup so you can steer.

16. Question 16 of 25

    In a dual air brake system, which brakes does the primary system usually run?

    • AThe front brakes
    • BThe rear brakes
    • COnly the parking brakes
    • DOnly the trailer brakes
    Show answer

    Answer: B. The rear brakes

    The primary system usually works the rear axle brakes. The secondary works the front brakes and maybe one rear axle.

17. Question 17 of 25

    Before driving a vehicle with a dual air system, how much pressure should the compressor build in both systems?

    • AAt least 150 psi
    • BAt least 40 psi
    • CAt least 100 psi
    • DAt least 60 psi
    Show answer

    Answer: C. At least 100 psi

    Let the compressor build up at least 100 psi in both the primary and secondary systems before driving.

18. Question 18 of 25

    In a dual air system, how quickly should pressure build from 85 to 100 psi at operating rpm?

    • AWithin about 3 minutes
    • BWithin about 45 seconds
    • CWithin about 5 seconds
    • DWithin about 10 minutes
    Show answer

    Answer: B. Within about 45 seconds

    In dual systems, pressure should go from 85 to 100 psi within 45 seconds. Larger tanks may take longer; check the specifications.

19. Question 19 of 25

    What is the main job of the slack adjuster?

    • AIt tells the compressor when to pump
    • BIt drains water from the air tanks
    • CIt turns on the brake lights when braking
    • DIt links the push rod to the brake camshaft
    Show answer

    Answer: D. It links the push rod to the brake camshaft

    The push rod turns the slack adjuster, which turns the camshaft and S-cam to apply the brakes.

20. Question 20 of 25

    You pull hard on a slack adjuster with the brakes released. How far can it move before it probably needs adjusting?

    • AIt should never move at all, even slightly
    • BAbout one inch where the push rod attaches
    • CAbout one-eighth inch at the end of the arm
    • DAbout six inches at the end of the arm
    Show answer

    Answer: B. About one inch where the push rod attaches

    If a slack adjuster moves more than about one inch where the push rod attaches, it probably needs adjustment.

21. Question 21 of 25

    What must you do before checking slack adjusters?

    • ASet the parking brakes and run the engine
    • BRaise the vehicle on a jack so wheels turn
    • CDrain the air tanks completely first
    • DChock the wheels and release the parking brakes
    Show answer

    Answer: D. Chock the wheels and release the parking brakes

    To check slack adjusters you must release the parking brakes, so chock the wheels first on level ground.

22. Question 22 of 25

    What is the most common problem found in roadside inspections of air brakes?

    • AMissing reflective triangles
    • BBrakes out of adjustment
    • CEmpty alcohol evaporators
    • DBroken brake lights
    Show answer

    Answer: B. Brakes out of adjustment

    Out-of-adjustment brakes are the number one problem inspectors find.

23. Question 23 of 25

    Your vehicle has automatic slack adjusters. Do they still need to be checked?

    • ANo, they adjust themselves
    • BOnly once a year at service
    • CYes, during every inspection
    • DOnly after a crash or a fault
    Show answer

    Answer: C. Yes, during every inspection

    Vehicles built since 1994 have automatic slack adjusters, but they must still be checked.

24. Question 24 of 25

    An automatic slack adjuster is out of adjustment. What does this usually mean?

    • AThere is probably a mechanical problem that needs repair
    • BThe tanks need draining
    • CThe governor is set too high
    • DIt just needs a quick manual adjustment and no repair
    Show answer

    Answer: A. There is probably a mechanical problem that needs repair

    Manual adjustment of an automatic adjuster is only a temporary emergency fix. Have the vehicle repaired as soon as possible.

25. Question 25 of 25

    How long can a crack in a brake drum be?

    • AAny length, as long as it is not leaking
    • BUp to the full width of the friction area
    • CUp to half the width of the friction area
    • DUp to 6 inches, measured along the drum
    Show answer

    Answer: C. Up to half the width of the friction area

    Drum cracks must not be longer than one-half the width of the friction area.

Keyboard: 1–4 to answer, Enter for the next question.

The real test: Varies by state; usually 25 questions, with 20 correct needed (80%)