Braking systems and suspension
Braking systems and suspension
This domain tests hydraulic braking, friction components, ABS, and suspension support components. These are safety-critical systems: every repair ends with leak, pedal, and road tests.
Core rules and concepts
- The hydraulic brake system uses incompressible fluid. A spongy pedal means air, a leak, or master cylinder bypass.
- DOT 3 and DOT 4 glycol fluids absorb moisture; DOT 5 silicone is not interchangeable. Use the vehicle specification.
- Bleed farthest wheel from the master first, commonly RR, LR, RF, LF; keep the reservoir full and do not let it run dry.
- Pads: check minimum thickness, rotor thickness, runout, scoring, and wear pattern.
- Suspension: worn shock absorbers cause bounce and uneven tire wear; check ball joint play with the wheel unloaded; inspect springs, bushings, and mounts.
- ABS warning: scan wheel speed sensor codes; do not open high-pressure lines without depressurizing.
Worked example
A spongy pedal appears after pad replacement. Check fluid level and leaks. If the pedal pumps up, air is likely; bleed in the correct sequence and recheck. If the pedal sinks under steady pressure, suspect an internal master cylinder leak. Replace and bench-bleed if needed.
Traps that cost marks
Pushing the caliper piston back with the bleeder closed, which can force dirty fluid into the ABS unit; mixing DOT fluids; measuring pad thickness without the backing plate; checking a ball joint with the suspension loaded; and not torqueing wheel nuts evenly.