Hydraulic Brake System Diagnosis and Repair
What a school-bus technician must know
Hydraulic systems use a master cylinder, lines, hoses, calipers or wheel cylinders, and often a proportioning or combination valve. Understand the master cylinder as two pistons in tandem: the primary feeds rear brakes, the secondary front (or as designed). A bypassing master cylinder causes a slowly sinking pedal; a leaking one causes fluid loss and a soft pedal. Know DOT 3/4 glycol fluid versus DOT 5 silicone-never mix, and know that glycol fluid absorbs moisture.
Concepts to master
Brake fade (heat), pull (dragging caliper or collapsed hose), pedal pulsation (warped rotor or excessive runout), and spongy pedal (air in system). Understand bleeding sequence and that a collapsed flexible hose can act as a one-way check valve, causing a dragging brake. Know residual pressure valves and metering valves on older systems. Recognize that a low fluid level with no visible leak often means a worn wheel cylinder or caliper piston seal.
Traps in questions
- Blaming the master cylinder for a pull-pulls are usually a caliper, hose, or contamination issue.
- Forgetting that a dragging brake can overheat and cause a pull or fire risk.
- Assuming a soft pedal is always air; a bypassing master also causes it.
- Mixing DOT fluid types.
Rule of thumb
A pedal that sinks slowly while held = internal master cylinder bypass; a pedal that is spongy = air; a pedal that is low but firm = rear adjustment or a seized component.