CDL Air Brakes Practice Test

What this section covers

• Compressor, governor, tanks, and the dual air system
• Low-air warning devices and the 60 psi threshold
• Spring parking brakes and why air pressure holds them off
• Static and applied leakage tests and their allowable psi per minute
• Brake lag, stopping distance, and brake fade on grades
• Slack adjusters and who is allowed to adjust them

10 practice questions with explanations

1. 1. The air compressor governor controls:

   • A. Engine speed
   • B. When the compressor pumps air into the tanks — correct
   • C. How fast the trailer brakes apply
   • D. The parking brake

   Why: The governor cuts compressor pumping in and out based on tank pressure (about 100–125 psi).

2. 2. Low-air warning must activate before pressure drops below:

   • A. 100 psi
   • B. 90 psi
   • C. 60 psi — correct
   • D. 40 psi

   Why: Federal rule: the low-air warning must come on before pressure falls below 60 psi.

3. 3. Spring (parking) brakes are held OFF by:

   • A. Air pressure — correct
   • B. Hydraulic fluid
   • C. Springs
   • D. Electric current

   Why: Air pressure compresses the springs. When air is lost, the springs apply the brakes.

4. 4. During the applied-leakage test with the engine off, brakes applied, service brake air loss must not exceed:

   • A. 1 psi in 1 min (single) / 3 psi in 1 min (combo)
   • B. 3 psi in 1 min (single) / 4 psi in 1 min (combo) — correct
   • C. 5 psi any vehicle
   • D. 10 psi in 3 min

   Why: With the service brakes applied, max leakage is 3 psi/min single, 4 psi/min combo.

5. 5. You should never use spring parking brakes if:

   • A. The trailer is loaded
   • B. The service brakes are hot — correct
   • C. Air pressure is above 100 psi
   • D. The vehicle is uphill

   Why: Applying spring brakes over hot service brakes can trap heat and damage drums.

6. 6. Brake fade is caused by:

   • A. Cold rotors
   • B. Excessive heat from overuse of service brakes — correct
   • C. Low tire pressure
   • D. Too much air in the tanks

   Why: Riding the brakes downhill overheats the drums and reduces stopping power.

7. 7. Air brakes take longer to stop than hydraulic because:

   • A. Air is heavier
   • B. There is brake lag while air travels through the lines — correct
   • C. Drums are smaller
   • D. Air brakes have less friction

   Why: Brake lag (about 0.4 sec) adds roughly 32 ft at 55 mph before braking begins.

8. 8. The air compressor is usually driven by:

   • A. The battery
   • B. Engine gears or a belt — correct
   • C. Exhaust pressure
   • D. The transmission output shaft

   Why: It's engine-driven, either gear or belt driven off the crankshaft.

9. 9. Manual slack adjusters should be adjusted:

   • A. By the driver anytime brakes feel soft
   • B. Only by a qualified mechanic — correct
   • C. Every fuel stop
   • D. Never

   Why: Only a certified brake mechanic should adjust slack adjusters.

10. 10. In a dual air system, if one tank loses pressure:

    • A. Both circuits fail
    • B. The other circuit still brakes some wheels — correct
    • C. The trailer applies fully
    • D. The truck stalls

    Why: Dual systems keep partial braking on the remaining circuit.

How to study this section

• Remember the direction: air pressure releases the spring brakes, loss of air applies them.
• Know the leakage limits cold — 3 psi per minute single vehicle, 4 psi per minute combination, with the brakes applied.
• The low-air warning must come on before pressure drops below 60 psi.
• Practice saying the air brake check out loud; several states test it during the pre-trip too.

Frequently asked questions

What happens if I don't take the air brakes test?

You get an air brake restriction on your CDL, which prohibits you from operating a commercial vehicle equipped with air brakes.

How many questions is the CDL air brakes test?

Typically about 25 questions with an 80% passing score, though it varies by state.