Parking Brake System Diagnosis and Repair
What a school-bus technician must know
School buses use either spring brakes (air) or mechanical parking brakes (cable/drum or driveline). For air buses, the spring brake chamber contains a large spring that applies when air is released; the parking brake valve (yellow diamond) exhausts air to apply and admits air to release. Understand the anti-compounding valve and double check valve so service and parking brakes don't fight. For mechanical systems, know the cable, equalizer, and lever adjustment and that a driveline parking brake must hold on a grade.
Concepts to master
The parking brake must hold the bus on a grade per the manufacturer and regulation; test by applying and attempting to move. Know that caging a spring brake (manual release bolt) is for emergency movement only and must be restored. Understand that a leaking spring brake diaphragm can cause drag or failure to release. Know the dash warning and that the parking brake must release fully before driving.
Traps in questions
- Confusing caging (manual release) with normal release; caging is temporary.
- Assuming a parking brake that won't release is a valve issue when a leaking chamber or stuck relay is the cause.
- Forgetting the anti-compounding function.
- Overlooking cable stretch and equalizer adjustment on mechanical systems.
Rule of thumb
If a spring brake won't release, check for air reaching the chamber first (valve/line), then the chamber itself; if it won't apply, suspect the spring or pushrod/caging bolt left engaged.