Disc Brake Diagnosis and Repair
Core concepts
A caliper piston seal does two jobs: it seals, and it retracts the piston (rollback) when pressure releases. Sliding/floating calipers must move freely on lubricated guide pins and bushings; a binding pin typically wears the inner pad faster and can cause a pull. Know the measurements: rotor minimum thickness (cast into the hat) measured with a micrometer at eight or more points, and lateral runout measured with a dial indicator with the wheel nuts torqued. Disc thickness variation - not "warping" - is the usual cause of pedal pulsation, and excess runout or hub/bearing play is the usual cause of thickness variation. Also know pad bedding, wear indicators, anti-rattle hardware, shims, and torque specs. Rear calipers with integral parking brakes often require the piston to be screwed in rather than pressed.
Traps
- Pulsation: measure runout and thickness variation and check hub/bearing endplay before replacing rotors.
- Inner pad worn more than outer on a floating caliper points to seized slides, not defective pads.
- Never let a caliper hang by the hose; support it, and watch reservoir level when retracting pistons.
- Directional rotors must go on the correct side; ventilation vanes curve a specific way.
- Some technicians open the bleeder when retracting a piston to avoid pushing contaminated fluid back into the HCU.
Rule of thumb
Minimum thickness is a discard dimension, not a machining target: if machining would leave the rotor at or below minimum, replace it. Use a micrometer for thickness and a dial indicator for runout.