Many cars also have power assist to reduce the effort required to brake.
Typically, the power source is the pressure difference between the partial vacuum in the inlet manifold and the outside air.
The auxiliary servo unit is provided with a pipe connection to the inlet manifold.
The linear motion servo is mounted between the brake pedal and the master cylinder. The brake pedal pushes a lever which in turn pushes the master cylinder piston.
But the brake pedal also works on a set of air valves, and there is a large rubber diaphragm connected to the master-cylinder piston.
When the brakes are off, both sides of the diaphragm are exposed to the vacuum from the manifold.
Pressing the brake pedal closes the valve that connects the rear side of the diaphragm to the manifold and opens a valve for outside air to enter.
The higher pressure of the outside air forces the diaphragm to push the master cylinder piston forward, thereby contributing to the braking force.
If the pedal is then held, and pressed no further, the air valve admits no more air from outside, so the pressure on the brakes remains the same.
When the pedal is released, the space behind the diaphragm is reopened to the manifold, so the pressure drops and the diaphragm falls back.
If the vacuum fails due to engine stop, for example the brake is still working because there is a normal mechanical connection between the pedal and the master cylinder. However, more force must be applied to the brake pedal to apply them.