Hydraulic Brake System Diagnosis and Repair
Layout and valves
The master cylinder creates pressure; the compensating port lets fluid return as it expands with heat. A combination valve may package a metering valve (delays front apply on some designs), a proportioning valve (limits rear pressure), and a pressure differential switch that lights the warning lamp if a circuit fails. Residual pressure valves hold about 10 psi for drum brakes and about 2 psi for disc; fitting a 10 psi valve to a disc circuit causes drag and heat.
Diagnosis by pedal feel
- Spongy or low pedal: air in the system, or misadjusted rear drum brakes.
- Pedal slowly sinks to the floor at constant pressure: internal leak past the master cylinder cups - no external leak will be found.
- Hard pedal, high effort: failed hydro-boost or vacuum booster, collapsed hose, or a seized caliper.
- Pulls to one side: restricted hose, sticking caliper, or contaminated pads.
Fluid and bleeding
DOT 3, DOT 4, and DOT 5.1 are glycol-based and hygroscopic. DOT 5 is silicone and must never be mixed with them, and is generally unsuitable for ABS systems. Bleed farthest-first, typically right rear, left rear, right front, left front. Bench-bleed a new master cylinder. Some ABS units require a scan tool to cycle the pump and valves for a complete bleed.
Inspection
Have an assistant hold pressure while you check hoses for cracks and ballooning; replace any suspect hose. Measure rotor thickness, pad wear, and verify caliper slide pins move freely.
Worked example: a truck has a firm pedal that fades to the floor while held at a light, with no fluid loss - the master cylinder cups are bypassing internally.
Rule of thumb: spongy means air; sinking means an internal leak; low means adjustment or drum wear.