Hydraulic System Diagnosis and Repair
Core concepts
A hydraulic brake system transmits force through an incompressible fluid, so every diagnosis begins with one question: is the fault in pressure generation, retention, or distribution? Know the master cylinder (including quick take-up and step-bore designs), the combination valve family - metering/hold-off, proportioning, and the pressure-differential switch - and residual pressure valves (roughly 10 psi for drum circuits, 2 psi or none for disc). Understand bleeding: bench bleeding a new master cylinder, the usual sequence (longest line first), and the fact that some ABS units require a scan-tool-actuated bleed. Know that DOT 5 silicone fluid must never be mixed with DOT 3/4/5.1 glycol fluid and is not intended for ABS systems.
Traps
- A pedal that slowly sinks under steady pressure with no visible leak is internal bypass in the master cylinder - not air.
- A spongy pedal indicates air; a low but firm pedal points to adjustment or pushrod length.
- A collapsed hose can act as a one-way valve, causing a dragging wheel or a pull that appears after braking.
- Mismatched flares (ISO bubble vs. double flare) leak and mimic a failed component.
- A residual pressure valve left in a disc circuit causes drag, heat, and premature pad wear.
Rule of thumb
Pump the pedal. If it rises and firms with each pump, you have air or excess clearance. If it stays low and gradually falls while held, suspect the master cylinder. Confirm with a pressure check at each wheel before condemning any valve.