How Does a Brake Booster Work?

by Contributor


The brake booster is situated between the brake pedal and the master cylinder. The body, booster piston, piston return spring, control valve mechanism, and the reaction mechanism, are the parts that make up the unit known as a brake booster. There are two chambers in the booster body: the constant and variable pressure chambers. Bridging the two chambers is a mechanism called a diaphragm. A control valve device manages the pressure within the variable pressure chamber.

Increased force

A brake booster increases the force applied by the brake pedal. The amount of increased force is dependent on the diaphragm. The pedal force is doubled (or quadrupled) by the use of atmospheric pressure and the manifold vacuum of the car engine. The piston is moved by a spring that is under pressure when the vacuum is working on both sides of the piston. What makes the brake booster work is this pressure difference acting on the components within the brake booster body.

Air valve off/ on

The air valve is mated to the valve operating rod. A return spring tugs the air valve to the right (the OFF position). In opposition, the control valve is moved over to the left by the control valve spring. The air valve contracts the control valve. As a result, the outside air passing into the air cleaner mechanism does not reach the variable pressure chamber. A passage between the constant pressure chamber and the variable pressure chamber is created. The passage allows the vacuum path into the variable pressure chamber, moving the piston to the right side of the chamber. The air valve moves away from the control valve, bringing outside pressure into the variable pressure chamber. The differential between the constant and variable pressure chambers moves the piston left. The reaction disc knocks the booster rod left, which produces braking force. The air valve is in the ON position when the brake pedal is depressed. The valve operating rod presses the air valve to the left side. The control valve makes contact with the vacuum valve, which blocks the passage between the constant pressure chamber and the variable pressure chamber.

Operation in action

In action, when the pressure is put on the pedal, the same pressure is transferred to the booster air valve. The master cylinder applies the brakes in response to pressure from the brake booster. In essence, atmospheric air enters the piston chamber, causing the piston to push against the spring (compressing the spring). The piston moves left, and the pressure moves the piston rod. This pressure then causes the piston rod to move against the master cylinder's piston. When the piston in the master cylinder moves, the result is the production of hydraulic power being applied by the brake booster to the vehicle's braking system.

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