A drum brake has a hollow drum that turns with the wheel. Its open back is covered by a stationary backplate on which there are two curved shoes carrying friction linings.
The shoes are forced outwards by hydraulic pressure moving pistons in the brake's wheel cylinders , so pressing the linings against the inside of the drum to slow or stop it.
Each brake shoe has a pivot at one end and a piston at the other. A leading shoe has the piston at the leading edge relative to the direction in which the drum turns.
The rotation of the drum tends to pull the leading shoe firmly against it when it makes contact, improving the braking effect.
Some drums have twin leading shoes, each with its own hydraulic cylinder; others have one leading and one trailing shoe - with the pivot at the front.
This design allows the two shoes to be forced apart from each other by a single cylinder with a piston in each end.
It is simpler but less powerful than the two-leading-shoe system, and is usually restricted to rear brakes.
In either type, return springs pull the shoes back a short way when the brakes are released.
Shoe travel is kept as short as possible by an adjuster. Older systems have manual adjusters that need to be turned from time to time as the friction linings wear. Later brakes have automatic adjustment by means of a ratchet.
Drum brakes may fade if they are applied repeatedly within a short time - they heat up and lose their efficiency until they cool down again. Discs, with their more open construction, are much less prone to fading.