Air Brake System Maintenance and Fault Diagnosis
What the exam tests
Maintenance and fault diagnosis questions test whether you can read symptoms, choose a logical diagnostic step and avoid repairs that hide the root cause.
Core rules and concepts
- Air-system contamination is a symptom, not a normal condition. Water and oil at drain cocks point to air dryer or compressor problems.
- A compressor that builds pressure slowly may have a worn compressor, slipping belt, restricted intake, leaking governor signal line or major system leak.
- A governor that will not unload can overpressure the system; one that will not load causes low pressure and frequent warnings.
- Automatic slack adjusters are not adjusted manually to correct excessive stroke. If stroke is out of limits, find the underlying wear or damage.
- Brake drag, uneven braking, poor release and spring brakes that will not release are diagnosed by checking chamber pressure, pushrod travel, slack adjuster condition, relay and quick-release valves, and kinked or collapsed hoses.
Worked example
A tractor pulls left when braking, and the right steer wheel is hotter than the left. The right brake chamber has normal pushrod stroke, but the right slack adjuster does not return fully. Reasoning: the chamber is applying, but the brake is not releasing completely. Check for a seized slack adjuster, collapsed return spring, restricted release port or kinked hose. Replacing the chamber without checking release would not fix the drag.
Traps that cost marks
Adjusting an automatic slack adjuster to compensate for worn foundation brakes; replacing a compressor because pressure is low without checking the governor and leaks; ignoring air dryer purge; and using sealant or tape that can enter the air system. Diagnose first, then repair.