Braking Systems
Braking is about converting motion into heat safely and predictably. This domain tests drum and disc brakes, hydraulic and air systems, brake fade, bleeding and the diagnosis of a poor pedal.
Types of brake
Drum brakes use two shoes pressed outwards against a rotating drum. A leading shoe is dragged into contact, so it is self-energising; a trailing shoe is not. Disc brakes use a caliper with pads clamping a rotating disc, which runs cooler and resists fade better. A parking brake is mechanically operated, usually on the rear wheels, and must hold the vehicle on a gradient independently of the hydraulic system.
Hydraulic operation
Pedal force acts on a master cylinder, pressure passes through pipes to wheel cylinders or calipers, and the fluid is the medium. Brake fluid is hygroscopic, absorbing moisture from the air, which lowers its boiling point; DOT 3 and DOT 4 glycol fluids are common, while DOT 5 is silicone based and must not be mixed with them. Air in the hydraulic circuit compresses and gives a spongy pedal, so the system must be bled, working from the wheel furthest from the master cylinder and keeping the reservoir topped up. Heavy vehicles use air brakes with compressors and reservoirs. Anti-lock braking systems release and reapply pressure at individual wheels to keep them rolling.
Fade and diagnosis
Brake fade is the loss of friction from overheating, common with repeated heavy braking on long descents; it is avoided by using the gears for engine braking. A pedal that sinks slowly to the floor under steady pressure indicates a fluid leak or a faulty master-cylinder seal, not air. Pulling to one side points to a seized caliper, a contaminated lining or a collapsed hose.
Worked example
After a recent repair, the pedal feels soft and can be pressed almost to the floor, but the fluid level is steady. Reasoning: fluid level steady means no external leak, and the springy feel indicates air. Bleed the circuit in the correct sequence, starting at the wheel farthest from the master cylinder, and top up with the specified fluid.
Traps that cost marks
Mixing DOT 5 silicone fluid with DOT 3 or 4; ignoring brake fluid changes; bleeding in the wrong order; using the handbrake as a service brake.