Air brakes practice test
Air brakes aren’t an endorsement — they’re a restriction you avoid. Skip the air brakes knowledge test (or test in a vehicle without them) and your CDL carries an L restriction barring you from air-braked vehicles. Since nearly every real truck has air brakes, treat this test as mandatory.
An air brake system is really three different systems. They are:
How the air brakes test works
To drive a vehicle with air brakes you must pass the air brakes knowledge test AND take your skills test in a vehicle equipped with air brakes. Otherwise the state prints an L restriction (or M/N variants for partial systems) on your license.
This is the most figure-heavy section of the CDL manual. The test loves exact numbers, so drill these until they’re automatic:
- Governor: cuts the compressor out around 125 psi, back in around 100 psi
- Low air warning: must activate before pressure drops below 60 psi
- Spring brakes: apply themselves at 20–45 psi
- Pressure buildup: 85 → 100 psi within 45 seconds (dual systems)
- Static leak limits: under 2 psi/min single vehicle, 3 psi/min combination (engine off, brakes released)
- Applied leak limits: under 3 psi/min single, 4 psi/min combination (pedal held)
- Safety valve: opens at about 150 psi
The seven-step air brake check
Expect at least one question on the pre-trip air brake test sequence: build full pressure, shut the engine off, release the service brake and time the static leak rate, then apply the pedal and time the applied leak rate, test the low-pressure warning by fanning the pedal down, keep fanning until the spring brakes pop out, then restart and rebuild pressure. It’s also exactly what the examiner wants to see on your skills test.
Frequently asked
- Is air brakes an endorsement?
- No — it works in reverse. Everyone is assumed to need it; failing to test on air brakes puts an L restriction on your CDL that blocks you from driving air-braked vehicles.
- How do I remove an L restriction later?
- Pass the air brakes knowledge test and retake the skills test in a vehicle with a full air brake system. Your state DMV updates the license.
- How many questions are on the air brakes test?
- Usually about 25 questions, with roughly 80% needed to pass. Nearly all of them involve the specific psi figures and test procedures covered above.