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Free CDL Practice Test Air Brakes 2026

Critical for Class A & B

If you fail this section, you get an "L" restriction — no air brake vehicles.

AB

Virtually all commercial trucks and buses use air brakes. Pass this test or your CDL will have an "Air Brake Restriction (L)" permanently stamped on it.

  • Required for Class A and most Class B
  • Fail = permanent L restriction
  • 25 questions, 80% to pass

Try The 2026 Air Brakes Practice Test

Question 1 of 25

The air compressor governor cuts out at what PSI?

A. 100 PSI
B. 120 PSI
C. 125 PSI (Tap to verify)
 Question 2 of 25

At what PSI should the low air warning activate?

A. 60 PSI (Tap to verify)
B. 45 PSI
C. 80 PSI
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Air Brakes "Cheat Sheet": Critical PSI Numbers

The air brakes test loves to quiz you on specific numbers. Memorize this table:

Event PSI
Governor cut-in100 PSI
Governor cut-out125 PSI
Low air warning buzzer60 PSI
Spring brakes engage20-45 PSI
Build-up time (85-100 PSI, dual air system)≤ 45 sec
Static leak, brakes released (single vehicle)2 PSI / min
Static leak, brakes released (combination)3 PSI / min
Applied leak, pedal held (single vehicle)3 PSI / min
Applied leak, pedal held (combination)4 PSI / min

The L.A.B. Pre-Trip Check

L.A.B. stands for Leaks, Alarm, Button. It is a memory aid for the in-cab air brake check; your state may name or order these steps differently:

  1. LLeaks: With a full system, turn off the engine, release the parking brakes, and hold the brake pedal. After the first drop, pressure should not fall more than 3 PSI/min (single vehicle) or 4 PSI/min (combination).
  2. AAlarm: Fan the brake pedal to lower the pressure. The low air warning light or buzzer must come on before pressure drops below 60 PSI.
  3. BButton: Keep fanning. The parking brake valve should pop out between 20 and 45 PSI as the spring brakes come on.

Also check the build-up rate: with the engine at operating rpm, pressure should rise from 85 to 100 PSI within 45 seconds in a dual air system. A slower build-up points to a compressor or leak problem.

Dual Air Brake System

Modern trucks have two independent air systems (primary and secondary). If one fails, the other provides enough braking to stop safely. The system is not a backup — both operate simultaneously, each controlling different sets of brakes.

Frequently Asked Questions

What is the L.A.B. air brake check?
L.A.B. stands for Leaks, Alarm, and Button. It is a memory aid for the in-cab air brake check: an applied leak test, the low air warning, and the parking brake valve popping out. Your state may name or order these checks differently.
At what PSI do air brakes lock up?
The spring brakes (parking brakes) automatically engage below 20-45 PSI. The low air warning buzzer must activate at or before 60 PSI.
How many questions are on the Air Brakes test?
Most states have 25 questions. You need at least 80% (20 correct) to pass. If you fail, you get an "L" restriction on your CDL.

Air Brakes

Don't Get the L Restriction

Master PSI numbers, the L.A.B. check, and dual air systems with our interactive test.

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