What is a Vacuum Servo?

Vacuum servos are devices that are used in many cars and trucks to reduce the effort required to activate the brakes. This component, which is also known as a vacuum brake booster, consists of a large vacuum reservoir that is divided into two compartments. One end of the device is connected by linkage to a brake pedal and the other end consists of a plunger that is in contact with the brake master cylinder. When the driver depresses the brake pedal, atmosphere enters one of the compartments in the reservoir. This pressure differential magnifies the force that was applied via the brake pedal, so less effort is required from the driver than would be if the vehicle lacked a vacuum servo.

Internal combustion engines generate vacuum as a natural part of the combustion process. This is typically referred to as manifold vacuum, and it may be used to power components such as windshield wiper motors and vacuum bake boosters. Vehicles that use diesel or electrical motors may also use a vacuum servo by utilizing a component known as a vacuum pump. These devices are either mechanically or electrically powered and work by pumping the air out of a sealed container.

Early automotive braking systems used direct, mechanical connections between brake pedals and master cylinders. This type of system uses only the force available from the driver's foot to depress the master cylinder to activate the brakes. As a result, a great amount of force is often required and it may take some time for pressure to build up and the brakes to activate. A vacuum servo not only reduces the effort required to activate the master cylinder, but also typically provides full braking power as soon as the pedal has been depressed a certain distance.

Vacuum brake boosters operate by creating and then exploiting a pressure differential. A typical vacuum servo consists of two sealed vessels, one inside the other. Vacuum is applied to remove air from inside both containers, and a check valve typically keeps atmosphere from accidentally reentering.

Typical vacuum servo designs also include a diaphragm between the two sealed vessels. Air is allowed to enter one of the chambers as the brake pedal moves downwards, which create a pressure differential and moves the diaphragm. Additional air can enter the device as long as the pedal is depressed, creating a larger pressure differential and more braking force.

Some vehicles, particularly large combination trucks, use air brakes that do not require a vacuum servo. Other vehicles use a hydraulic brake booster, regardless of whether vacuum is readily available. One particular application uses the same pressurized fluid to operate the power steering rack and provide a boosting force to the brakes.