Critical for Class A & B
If you fail this section, you get an "L" restriction — no air brake vehicles.
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
The air compressor governor cuts out at what PSI?
At what PSI should the low air warning activate?
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-in | 100 PSI |
| Governor cut-out | 125 PSI |
| Low air warning buzzer | 60 PSI |
| Spring brakes engage | 20-45 PSI |
| Build-up time (85-100 PSI) | ≤ 2 min |
| Max leakage (single vehicle) | 3 PSI / min |
The L.A.B. Pre-Trip Check
L.A.B. stands for Leakage, Applied pressure, Build-up time. This is the standard 3-step air brake test:
- LLeakage test: Build to governor cut-out. Turn off engine. Hold brakes. Loss must be ≤ 3 PSI/min (single) or ≤ 4 PSI/min (combination).
- AApplied pressure: With full service brake applied, air pressure should hold steady — if it drops quickly, you have a brake chamber leak.
- BBuild-up time: Start the engine. Air must build from 85 to 100 PSI within 2 minutes. If slower, the compressor is failing.
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? ▾
At what PSI do air brakes lock up? ▾
How many questions are on the Air Brakes test? ▾
Air Brakes
Don't Get the L Restriction
Master PSI numbers, the L.A.B. check, and dual air systems with our interactive test.