Braking
Braking
This domain tests hydraulic brake principles, drum and disc construction, bleeding, adjustment, ABS and parking brake operation. Questions often ask why a pedal is spongy, or the correct bleed order.
Core rules
- Hydraulic principle: pressure is transmitted equally in a closed system (Pascal). A tandem master cylinder has two circuits so one failure still leaves braking on two wheels.
- Disc brakes: caliper, pads, disc; self-adjusting and better heat dissipation. Drum brakes: wheel cylinder, shoes, return springs; self-energising but prone to fade and need adjustment.
- Brake fluid is hygroscopic; use the specified DOT grade from a sealed container, never mineral oil. Bleed air out starting at the wheel farthest from the master cylinder and work towards it - on most right-hand-drive cars, left rear, right rear, left front, right front.
- ABS prevents wheel lock by modulating pressure; it does not shorten stopping distance on loose gravel.
- Parking brake: mechanical cable or electronic; check travel and hold on a slope.
Worked example
A car has a spongy pedal that sinks slowly when held. Reasoning: a sinking pedal points to an internal leak past the master-cylinder seals, while a spongy pedal points to air in the hydraulic circuit. First check for leaks and fluid level, then pressure-bleed the system starting at the farthest wheel, keeping the reservoir topped up. If the pedal still sinks with no external leak, the master cylinder needs overhaul or replacement. Do not simply adjust the rear shoes - that would not fix a hydraulic fault.
Traps
- Bleeding the nearest wheel first, leaving air in the long line.
- Mixing DOT 5 silicone fluid with DOT 3 or 4.
- Letting the reservoir run dry during bleeding and drawing in more air.
- Measuring pad thickness without checking the disc for scoring and minimum thickness.