The Braking System, Disc vs Drum

Press the pedal and here's the path: modern brakes are hydraulic — the pedal's force travels through sealed brake fluid to the wheels, where it clamps friction material against a rotating part. The system is closed, so fluid can't compress or escape and pedal force becomes clamping force losslessly. A brake servo sits between your foot and the hydraulics, multiplying your effort — which is why the pedal goes heavy when the engine is off: the servo only works with the engine running (a heavy pedal with the engine off is the servo, not a fault).

At the wheels, two designs — and the difference is all about heat:

• Disc brakes — an open, exposed disc gripped by a caliper. Standard on modern cars, at least on the front axle. The open design sheds heat well.
• Drum brakes — a closed drum with shoes pressing outward inside it. Common on the rear axles of older and budget vehicles. The closed design traps heat — remember that word; it's the setup for the next sub-topic.