Air Brake System Maintenance, Leaks, and Low-Pressure Tests
This domain tests routine maintenance and the specific pressure tests used to decide whether an air brake system is safe: leak testing, low-pressure warning testing, compressor build-up, and governor and safety-valve behaviour.
Routine maintenance
- Drain all reservoirs daily (or at the end of each shift) to remove water and oil; in humid or freezing conditions, draining prevents ice in lines.
- Maintain the air dryer and, where fitted, the alcohol evaporator.
- Inspect hoses, fittings, chambers, and valves for leaks; use soapy water - bubbles show the source.
- Check slack adjusters and push-rod stroke against the manufacturer's limit; automatic slack adjusters should be diagnosed, not simply cranked.
- Any repair to the air system should be done by a qualified mechanic; the driver's job is inspection and reporting.
The pressure tests
- Compressor build-up: engine at fast idle, 85 to 100 psi within 45 seconds.
- Governor: cuts out near 125 psi and back in near 100 psi; the safety valve opens around 150 psi.
- Static leak: engine off, fully charged, parking brake released - 3 psi/min single, 4 psi/min combination.
- Applied leak: firm brake application held - 4 psi/min single, 6 psi/min combination.
- Low-pressure warning: fan the brakes; the warning must come on before pressure drops below 60 psi.
- Spring brakes: continue fanning; they should apply between 20 and 45 psi.
Worked example
After charging to 120 psi and shutting down the engine, a driver fans the brakes. The buzzer sounds at 50 psi. That is a failure: the warning must activate before the gauge falls below 60 psi, giving the driver time to stop before the spring brakes apply. The vehicle should be written up.
Traps
Testing with the engine running (masks leaks), testing with the parking brake set, accepting a warning at exactly 60 psi, and confusing the governor cut-out with the safety-valve setting.