About the Air Brakes Test
Almost every tractor-trailer, bus, and large straight truck runs air brakes, so the L restriction is one you do not want on your license. To clear it you pass the Air Brakes knowledge test, then take your skills test in a vehicle that has air brakes.
The test covers how an air brake system actually works: the compressor and governor, the dual air system, the supply and service circuits, slack adjusters, the low-air-pressure warning, and the spring brakes that lock the wheels if pressure drops. You'll also be tested on the proper way to inspect the system and run an applied-pressure leakage check.
What Test-Takers Miss Most
- 1Parts of the air brake system — The compressor, governor, reservoirs, and slack adjusters all show up by name. Know what each one does, not just that it exists.
- 2The leakage and warning checks — The pressure-drop limits during the applied test, and at what PSI the low-air warning and spring brakes activate, are exact numbers people guess on.
- 3Brake fade from overuse — Why riding the brakes downhill causes fade, and why engine braking and proper gear selection prevent it.
Choose Your State
Each state's test reflects its own CDL handbook, question count, and passing score. Pick yours to start a free Air Brakes practice test built for 2026.