Some vehicles have an indirect servo system installed in the hydraulic line between the master cylinder and the brake. This unit can be mounted anywhere in the engine compartment without having to be directly in front of the pedal.
It also relies on manifold vacuum to provide reinforcement. Pressing the brake pedal causes the master cylinder to generate hydraulic pressure, and the valve opens and triggers the vacuum servo.
The drum brake has a hollow drum that rotates with the wheel. Its open back is covered by a stationary backplate on which there are two curved shoes carrying friction linings.
The shoe is pushed outward by the hydraulically moving piston in the brake wheel cylinder, thus pressing the liner 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 end. The leading shoe has a piston at a leading edge with respect to the direction of rotation of the drum.
The rotation of the drum tends to pull the shoe firmly onto it upon contact, thereby improving the braking effect.
Some drums have two leading shoes, each with its own hydraulic cylinder; others have a leading shoe and a trailing shoe - there is a pivot on 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.
The adjuster shortens the stroke of the shoe as much as possible. Older systems have manual adjusters that need to be rotated from time to time as the friction lining wears. Later brakes are automatically adjusted by the ratchet.
If the drum brakes are used repeatedly for a short period of time, they may fade - they will heat up and lose efficiency until they cool again. The disc structure is more open and not easy to fade.