Diagnosis and Troubleshooting
Core concepts
Diagnosis is systematic: verify the complaint, gather information, inspect, test, and confirm the repair. Know the common air brake complaints and their causes: slow build (leaks, compressor, dryer), low pressure warning (leak, governor, compressor), dragging brakes (slack adjuster, anti-compound, sticking relay or treadle), and uneven braking (foundation brake wear, contaminated linings, seized components).
Use leak-down testing: with the system charged and engine off, apply and hold the brakes and watch for pressure drop; also check for static leaks with brakes released. Know that a leak can be internal (valve) or external (line, fitting, chamber). Understand the value of the soap-bubble test and listening for leaks.
Traps in questions
- Replacing parts before verifying the complaint and checking for leaks.
- Missing an internal leak in a valve because no external bubbles appear.
- Ignoring that a low-pressure warning can be caused by a faulty switch rather than actual low pressure.
- Overlooking driver-reported symptoms that point to a specific circuit.
Worked example
Customer reports the low-pressure buzzer sounds intermittently. Charge the system, shut down, and watch the gauge; if pressure holds, suspect the warning switch or wiring. If pressure drops, isolate circuits with the parking valve and section valves to find the leaking circuit, then soap-test fittings and chambers.