Air Brake System Diagnosis and Repair
What a school-bus technician must know
Air brakes on a school bus are a stored-energy system: a compressor, governor, reservoirs, a foot valve, relay valves, spring (parking) chambers, and slack adjusters. Understand the charging and application circuits as separate paths, and know the role of the supply reservoir (feeds the foot valve) versus the primary/secondary circuits and the parking/emergency reservoir. The governor cuts the compressor in near 85 psi and out near 105–125 psi; a bus that won't build pressure can be a failed compressor, stuck unloader, leaking line, or a governor fault.
Concepts to master
Know the low-pressure warning (must activate before 60 psi), the spring brake applying when pressure drops, and the safety valve protecting each reservoir. Understand slack adjuster function and the pushrod stroke method: a stroke beyond the manufacturer's limit means the brakes are out of adjustment or a component is worn. Know that relay valves shorten application time on long wheelbases, and that quick-release and ratio valves affect release timing.
Traps in questions
- Confusing a leak that only appears with brakes applied (application side) with one present at rest (supply side).
- Assuming low pressure means a bad gauge; verify with a test gauge.
- Forgetting that air dryers and alcohol evaporators address moisture, not leaks.
- Mixing up spring brake (parking/emergency) with service brake chambers.
Rule of thumb
If pressure builds slowly but holds steady when the engine is off, suspect compressor or governor; if it builds but drops with the engine off, suspect a leak-isolate by circuit with the reservoir drain valves.