Inspecting and Testing Air Brake Systems (leak, low pressure warning, spring brake check)
Inspecting and Testing Air Brake Systems
This domain tests the pre-trip air-brake tests: leakage, low-pressure warning, compressor build-up, and governor operation. The exam expects a sequence and numeric limits, not just “check the brakes.”
Test sequence and limits
- Park on level ground, chock wheels, and build pressure to governor cut-out (about 125 psi). Shut the engine off.
- Static leak test: with full pressure, engine off, release the parking brake, apply and hold the service brake. Watch the gauge for one minute. Loss should be no more than 3 psi for a single vehicle or 4 psi for a combination. A larger drop means a leak; do not drive.
- Low-pressure warning test: with engine off and ignition on, fan the service brake. The warning light/buzzer must activate before pressure falls below 60 psi. Continue fanning; spring brakes should apply between 20 and 45 psi.
- Spring brake check: with normal pressure, apply the parking brake and try to move gently in low gear; the vehicle should not move. Also verify the spring brakes apply automatically as pressure drops. Never crawl under a vehicle held only by air.
- Compressor/governor check: start the engine, build pressure, and confirm cut-out and cut-in. Pressure should build adequately; if build-up is slow, check the belt, compressor, or leaks.
Worked example
After charging to 125 psi, you shut off the engine, release the parking brake, and hold the service brake. In one minute the gauge falls from 125 to 121 psi on a combination vehicle. That is a 4 psi loss-at the limit, not a pass with margin. Recheck connections and treatment before dispatch; a 5 psi loss would fail.
Traps
Do not perform the leak test with the parking brake applied, and do not accept a warning that first sounds at 50 psi. Do not confuse spring-brake application pressure (20–45 psi) with warning pressure (below 60 psi).