Braking Systems
Hydraulics and friction
Braking systems convert kinetic energy to heat. Candidates must understand Pascal's principle, master and wheel cylinders, disc and drum brakes, brake fluid, bleeding, and ABS. Hydraulic pressure is transmitted equally in all directions, so a small force on a large master-cylinder piston creates a larger force at the wheel cylinders. Disc brakes use a caliper and pads; drum brakes use shoes and a wheel cylinder. Brake fade occurs when heat reduces friction, especially with overheated drums or contaminated fluid. ABS prevents wheel lock by modulating pressure, but it does not shorten all stopping distances on loose surfaces.
Traps in questions
A frequent trap is using the wrong brake fluid or mixing DOT types. Another is bleeding in the wrong order or allowing the reservoir to run dry. Questions may confuse brake fade with air in the system: air gives a spongy pedal, while fade gives a hard pedal with poor stopping. Do not overlook a seized caliper slide or contaminated pads as causes of pulling. The parking brake must be adjusted after rear brake service.
Rule of thumb
Bleed from the wheel farthest from the master cylinder first, usually rear right, rear left, front right, front left, keeping the reservoir topped up.
Worked example: a driver applies 200 N to a brake pedal with a 4:1 pedal ratio, producing 800 N at the master cylinder. If the master piston area is 5 cm² and a wheel cylinder area is 20 cm², force multiplication is 4:1, giving 3,200 N at that wheel cylinder.