Hydraulic Brake Systems
Core concepts
Some transit buses and support vehicles use hydraulic brakes, so know the hydraulic system: master cylinder, proportioning/combination valve, metering valve, residual pressure check valves (where used), wheel cylinders or calipers, and the hydraulic booster or hydro-boost that uses power steering pressure on many buses. Understand Pascal's law and how pedal force is multiplied by bore ratios. Know the difference between a split (diagonal or front/rear) system and how a failure affects pedal feel and braking.
Understand brake fluid: hygroscopic nature, boiling point, DOT ratings, and why contaminated or aerated fluid causes a low/spongy pedal. Know bleeding procedures and the purpose of a bench-bleed on a new master cylinder.
Traps in questions
- Confusing a spongy pedal (air in system) with a low pedal (rear brake adjustment or fluid loss).
- Forgetting hydro-boost uses power steering fluid, not brake fluid, and that a failed hydro-boost changes pedal effort.
- Assuming a proportioning valve fixes a rear-lockup complaint caused by contaminated fluid or seized caliper.
- Overlooking that DOT 3/4/5.1 are glycol-based and DOT 5 is silicone; mixing is a problem.
Rule of thumb
A pedal that pumps up and feels better usually points to air or a misadjusted rear brake; a pedal that slowly sinks under steady pressure points to an internal or external leak.