Braking systems
Braking systems
The paper covers hydraulic principles, drum and disc service, ABS, parking brake and fault diagnosis.
Core rules
- Hydraulic pressure is transmitted equally; a larger wheel cylinder or caliper piston gives more force. Pedal ratio multiplies driver force.
- Brake fluid is hygroscopic; DOT 4 wet boiling point is much lower than dry. Flush by time, not colour alone.
- Bleed in the manufacturer's sequence, usually farthest from the master cylinder first. Never let the reservoir run dry.
- Disc runout is measured with a dial gauge, typically under 0.05 mm. Thickness variation causes pulsation; minimum thickness is stamped on the disc.
- ABS: wheel speed sensors, tone rings and hydraulic modulator. A dirty sensor or damaged ring can set a code; do not confuse it with a base brake fault.
Worked example
A pedal is spongy and sinks slowly when held. A pressure test shows the master cylinder cannot hold pressure. Bench-bleed and replace the master cylinder, then bleed the system in order. If the pedal is firm but the car pulls, suspect a seized caliper or collapsed hose, not air.
Traps
- Bleeding in the wrong order or letting air enter.
- Pushing the caliper piston back without opening the bleed screw, forcing dirty fluid into the ABS unit.
- Using mineral oil or petrol to clean brake parts.
- Ignoring a low fluid level, which may indicate a leak or worn pads.