Mechanical and Parking Brake Systems
Core concepts
Transit buses use spring brake chambers (maxi-brakes) for parking and emergency braking. Understand the construction: service chamber with a diaphragm, and a spring chamber with a large spring held off by air pressure. When air is released (parking valve applied or air loss), the spring applies the brake mechanically through a pushrod. Know the importance of caging a spring brake before removal and the danger of disassembling a spring chamber.
Understand automatic slack adjusters: how they maintain pushrod stroke, how to measure applied stroke against the manufacturer's specification, and why a broken or worn slack adjuster causes long stroke and poor braking. Know the parking brake interlock and the anti-compounding valve that prevents service and spring brakes from fighting each other.
Traps in questions
- Assuming you can back off a spring brake with the service adjustment; you must cage it or use the release bolt.
- Treating a long pushrod stroke as a slack adjuster problem when the foundation brake (S-cam, bushings, drum) is worn.
- Forgetting that automatic slack adjusters must be checked at the applied stroke, not free stroke, on many designs.
- Ignoring the anti-compound valve when diagnosing dragging brakes.
Rule of thumb
Always cage spring brakes before service and follow the manufacturer's stroke limits; if the applied stroke exceeds spec, inspect the foundation brake before adjusting the slack.