Maintenance, Troubleshooting, and Out-of-Service Criteria
This domain tests whether you can diagnose common air brake faults and recognize the conditions that place a vehicle out of service. Questions mix symptom-matching with the roadside inspection rules.
Common symptoms and causes
- Pressure will not build or builds slowly: worn or slipping compressor belt, faulty governor, frozen or restricted line, or a large leak.
- Pressure builds past cut-out: stuck governor; the safety valve vents near 150 psi as a backup.
- Oil in the tanks: compressor passing oil - an internal compressor fault, not normal.
- Brakes slow to release: kinked or plugged line, faulty relay or quick-release valve, collapsed hose.
- Trailer brakes will not release: closed tractor protection valve, bad glad hand seal, or a kink in the supply line.
Out-of-service conditions
- 20 percent or more of a vehicle's brakes defective (for example, 2 of 10) is an out-of-service condition; any steering-axle brake defect also places it out of service.
- Push rod stroke beyond the adjustment limit for that chamber size - the widely used rule is that a manual slack adjuster moving more than about an inch at the push rod needs attention.
- Air loss beyond the leak limits, an inoperative low pressure warning device, spring brakes that fail to apply in the 20–45 psi range, a defective safety valve, inability to build 85–100 psi in 45 seconds, or an inoperative parking brake.
- Cracked drums, contaminated linings, broken springs, missing components, and chafed or leaking hoses.
Worked example
A technician measures a push rod stroke of 2 inches on a chamber whose limit is 1¾ inches. Reasoning: the stroke exceeds the limit, so the brake is out of adjustment and the vehicle is out of service until corrected - not simply "noisy."
Traps that cost marks
Manually adjusting an automatic slack adjuster to hide a stroke problem; treating oil in the tanks as normal; assuming a vehicle passes because only one brake is defective on a multi-axle unit.