Air Brake System Maintenance and Troubleshooting
What the exam tests
This domain tests daily maintenance and common fault diagnosis. Expect questions on air-tank draining, air dryers, slack adjusters, leaks, and symptoms such as slow buildup or pressure loss.
Daily and routine care
Drain reservoirs daily to remove water and oil. Maintain the air dryer and replace its filter as needed. Check the compressor belt, hoses, fittings, chambers, and slack adjusters. Keep the warning system and gauges working. Automatic slack adjusters should not be manually readjusted as a substitute for repair; if adjustment is abnormal, find the cause.
Common symptoms
- Will not build pressure: compressor, governor, slipping belt, or large leak.
- Builds too slowly: leak, restricted line, worn compressor, or governor issue.
- Pressure drops: leak, open drain, faulty valve, or chamber.
- Brakes drag: slack adjuster, chamber, valve, or foundation brake problem.
- Spring brakes will not release: low air, inversion valve, relay valve, or chamber.
Worked example
A truck sits overnight and the gauge reads 60 psi. The compressor builds to 125 psi, but the leak test loses 10 psi per minute. The reasoning is that the compressor can build air, but the system cannot hold it; use soapy water at fittings, hoses, chambers, and valves to locate the leak. Do not just raise the governor setting.
Traps that cost marks
Do not ignore oil in the tanks or a failing air dryer. Do not adjust automatic slack adjusters to hide a defect. Do not use the safety valve as a pressure regulator. Do not diagnose a leak from the gauge alone.