What Is Brake Servo?

A brake servo, also known as a brake booster, is a device that intensifies the force applied to the brake pedal in a motor vehicle. Brake servo units were developed in the 1960s when the switch from drum brakes -- which naturally reinforce themselves -- to disc brakes made them necessary.

Negative Pressure

In vehicles with an Otto engine -- that is, a four-stroke internal combustion engine -- negative pressure in the intake pipe is often sufficient to operate the brake servo. In all other engine types, a vacuum pump is required to create negative pressure.

Construction

Essentially, a pneumatic brake servo consists of two chambers separated by a movable diaphragm. While the engine of a vehicle is running, negative pressure is directed through a one-way valve into both chambers and, provided the brake is not applied, the chambers are connected by channels.

Braking

When the brake is applied, a connecting rod closes the connection between the two chambers. According to the force with which the brake pedal is pressed, the pressure in one chamber relative to the other intensifies the braking force.