Air Brake System Tests
What this domain covers
This domain is the numbers test: cut-in, cut-out, leak limits, warning threshold, spring brake range, build-up time, and safety valve setting. Memorize each value and the condition it is measured under.
The numbers
- Governor cut-in: about 85 psi; cut-out: about 100-125 psi.
- Build-up: 85 to 100 psi within 45 seconds at fast idle.
- Static leak (engine off, brakes released): 2 psi/min single, 3 psi/min combination.
- Applied leak (brakes fully applied, engine off): 3 psi/min single, 4 psi/min combination.
- Low-pressure warning: before 60 psi.
- Spring brakes fully apply: 20-45 psi.
- Safety valve: about 150 psi.
Why the conditions matter
The engine must be off during leak tests, or the compressor will mask a leak by replacing lost air. Brakes must be fully applied for the applied test because charged chambers and lines are where many leaks show up. The warning and spring brake checks are done by bleeding pressure down with the pedal, not by opening a drain valve, so you can watch the gauge fall.
Worked example
A combination vehicle with a full system loses 5 psi in one minute with the brakes applied. The limit is 4 psi/min, so the test fails and the vehicle should not be dispatched until the leak is found and repaired.
Traps that cost marks
- Swapping the 2/3 and 3/4 limits between single and combination vehicles.
- Running the leak test with the engine idling.
- Assuming the warning and the spring brakes act at the same pressure.
- Measuring build-up time at curb idle instead of fast idle.