Braking Systems and Vehicle Safety
Domain 6: Braking Systems and Vehicle Safety
This domain tests hydraulic braking, friction components, parking brakes, ABS and safety systems. Brakes are safety-critical, so specification, cleanliness and verification matter.
Core rules
- Hydraulic pressure is transmitted equally in a closed system. A spongy pedal usually means air in the system, a leak or a failing master cylinder.
- Brake fluid is hygroscopic. Use the specified DOT fluid from a sealed container; do not reuse old fluid. Water contamination lowers the boiling point.
- Bleed in the manufacturer's sequence, usually starting farthest from the master cylinder. Do not let the reservoir run dry.
- Inspect pad thickness, disc thickness, disc runout, caliper slides, hoses and park brake travel. Compare with specification, not guesswork.
- ABS uses wheel-speed sensors and a hydraulic modulator. A warning lamp means ABS may be disabled, but base brakes should remain. Do not assume normal braking is unaffected.
- Safety systems include seat belts, airbags, tyre condition and lighting. Tyre tread depth below 1.6 mm is a legal and safety limit in many markets.
Worked example
The pedal slowly sinks when held under steady pressure. Reasoning: a steady sink indicates fluid is escaping internally or externally, not just air. Check for external leaks at calipers, lines and the master cylinder. If none are found, the master cylinder may be bypassing internally. Air usually gives a spongy pedal that may pump up; a steady sink is a stronger warning of pressure loss. The vehicle should not be released until the fault is repaired and the pedal holds.
Traps
- Reusing brake fluid or using the wrong DOT grade.
- Pushing contaminated fluid back into the reservoir when compressing calipers.
- Bleeding without checking the reservoir level.
- Measuring pad thickness only on the outer pad.
- Ignoring a brake warning lamp or a low pedal.