Braking Systems and Wheel Alignment
This domain covers hydraulic braking, disc and drum service, ABS, parking brakes and wheel alignment angles. Questions test Pascal's law, bleeding, fluid selection, pad and disc limits, and the effect of camber, caster, toe and thrust angle.
Core rules
- Hydraulic pressure is transmitted equally; pedal force is multiplied by the pedal ratio and servo. A spongy pedal usually means air in the hydraulic system or a faulty master cylinder.
- Brake fluid is hygroscopic. Use the specified DOT grade; DOT 4 is common. Never mix DOT 5 silicone fluid with DOT 3 or DOT 4.
- Bleed from the wheel farthest from the master cylinder, following the manufacturer sequence, and keep the reservoir topped up. Do not push caliper pistons back without opening the bleeder or checking the ABS procedure.
- Disc limits: measure pad thickness and disc thickness with a micrometer; check runout with a dial gauge. Drum brakes need correct shoe adjustment and clean, dry linings.
- Alignment: camber is vertical tilt, caster is steering-axis tilt, toe is wheel direction relative to centre. Set toe with the steering wheel centred and thrust angle correct.
Worked example
After pad replacement the pedal is spongy and travels further. The technician bleeds the rear-left, rear-right, front-left and front-right calipers, keeping the reservoir full. The pedal becomes firm. Reasoning: air is compressible, so trapped air absorbs pedal travel; bleeding removes it.
Traps
- Spilling brake fluid on paint and failing to rinse it immediately.
- Mixing DOT 5 silicone with DOT 4 fluid.
- Setting toe with an off-centre steering wheel.
- Replacing pads without checking disc thickness and runout.