Brake System Servicing
Domain 6 tests hydraulic braking: how pressure is generated, transmitted and released, plus the measurement that decides whether parts are serviceable.
Core rules
- Pascal's law: pressure is equal throughout a closed hydraulic system; master cylinder bore and caliper piston area set the force and travel.
- Use only the specified DOT 3 or DOT 4 glycol fluid, sealed and fresh; DOT 5 silicone is not interchangeable. Fluid absorbs moisture, so bleed and replace on schedule.
- Bleed from the farthest wheel to the nearest in the maker's sequence (commonly RR, LR, RF, LF), keeping the reservoir above minimum.
- Pedal free play and travel: too much free play means air or a worn master; a pedal that sinks under steady pressure means an internal leak.
- Pad friction material below the stated minimum, or rivets touching the rotor, means replacement; rotors are condemned by the minimum thickness stamped on the casting, scoring and lateral runout.
- Drums are condemned by maximum inside diameter; check wheel cylinder seepage and the self-adjuster.
- ABS: never push caliper pistons back with the bleeder closed on a system with an accumulator; keep sensors and tone rings clean.
Worked example
A front rotor measures 23.6 mm with a caliper, and the casting is stamped MIN 24.0 mm; dial-indicator runout is 0.12 mm against a 0.10 mm limit. Reasoning: below minimum thickness there is too little metal to absorb and shed heat, so the rotor will warp and fade - replace it, do not resurface. The excess runout would cause pulsation even with new pads, so check the hub face for rust scale and clean it before fitting the rotor, then torque the wheel and road test.
Traps
- Pushing contaminated fluid back up into the ABS unit.
- Twisting or stretching a brake hose to move the caliper aside.
- Mixing DOT 5 with DOT 4, or using fluid from a long-opened container.
- Fitting new pads without cleaning and greasing the slide pins, or without checking the hose for cracks.