Brakes (A5)

A spongy pedal usually means air in the system, while a pedal that slowly sinks under steady pressure indicates an internally leaking master cylinder.

Brake fluid is hygroscopic and absorbs moisture, which lowers its boiling point and warrants periodic replacement.

Bleed the system in the manufacturer's specified sequence, keeping the reservoir full to avoid introducing air.

Disc brakes use pads that clamp a rotor; drum brakes use shoes that press outward against a drum.

A brake pulsation is caused by rotor thickness variation or excessive runout; measure before machining or replacing.

Rotors must stay above the minimum (discard) thickness to dissipate heat and resist warping.

The power booster multiplies pedal force; a failed vacuum booster or check valve causes a hard pedal.

The proportioning valve limits rear pressure to prevent early rear lockup, and the metering valve balances front-to-rear timing.

A sticking caliper causes drag, overheating, uneven wear, and a pull toward that wheel.

ABS prevents wheel lockup to maintain steering control, using wheel-speed sensors and a hydraulic modulator.

A lost wheel-speed signal typically sets a code, disables ABS, and reverts to normal braking with the warning lamp on.

Some ABS units require a scan tool to bleed the hydraulic unit fully.

A parking brake that will not hold is often caused by a misadjusted or seized cable or caliper mechanism.

Grinding with a metal-on-metal feel indicates friction material worn past its limit, damaging the rotor or drum.

Diagnose by confirming the complaint, inspecting for leaks and wear, and testing hydraulic and mechanical components.