Brake master cylinder
Definition
The master cylinder converts the force from the pedal and brake booster into hydraulic pressure; the tandem design has two pistons in series feeding two separate circuits (front/rear or diagonal) so that a leak in one still leaves half the braking. Internal failure is a worn primary or secondary cup seal that lets fluid bypass the piston: the pedal sinks slowly to the floor under steady pressure at a standstill with no visible leak, or the brakes feel fine when pumped but weak on a single application. External leaks appear at the rear seal into the booster (fluid in the booster, falling reservoir level with no wet spots) or at the reservoir grommets. A master cylinder cannot be bled with a sinking pedal fault, and cheap fluid, moisture corrosion and a bore scored by debris are the usual causes; bench-bleed a new unit before fitting and never push the pedal to the floor on an old cylinder during bleeding because the piston enters the corroded unused part of the bore. No specific codes; related signs are a brake warning lamp from the level switch, pressure sensor plausibility codes C0131 in ESP cars and a long or sinking pedal that is not air in the lines.