What Is a Brake Booster?

by Cayden Conor

Brake boosters are used only on power brakes, not manual brakes. The booster's function is to give more braking power with minimal pressure on the brake pedal. If the booster is not working properly, you will still have brakes, but the brake pedal will be very hard to push.


A brake booster is what gives power brakes their power. It works on vacuum from the engine. The booster pushes on the master cylinder with the pressure applied to the booster by stepping on the brake. If the engine is off, it cannot get vacuum, therefore the brakes will be hard when the engine is off. This is noticeable when pushing a stalled vehicle and stepping on the brake.


The brake booster is made of five parts: the booster piston, body, booster return spring, control valve and reaction valve. The body has two chambers--the variable pressure chamber and constant pressure chamber--separated by a diaphragm.


The booster's size and whether it has a single or more than one diaphragm depend on the year, make and model of the vehicle. When ordering a new booster, tell the auto parts store or dealer the year, make and model of your vehicle, and they will give you the correct one.

Time Frame

Most car parts are replaced based on mileage. There is no set time for replacing a brake booster---it can last more than 30 years in many cases, but it can wear out earlier. Sometimes a master cylinder can cause a brake booster to go bad. If the master cylinder leaks on the inside of the brake booster (an uncommon occurrence), and the master is not replaced immediately, the booster will need to be replaced. The brake fluid causes the diaphragm to decompose.


If you have any problems with the brakes on your vehicle, get the entire brake system checked out immediately. Replace any worn out or broken parts. When one part of the brake system breaks or wears out, it affects other parts. For example, if the master leaks, it can cause booster failure. If the pads are worn down and not changed immediately, they will score the rotors to a point where they must be replaced. If pads are let go too long, the rotors will be too thin and it is possible to overextend the calipers.

About the Author

Cayden Conor has been writing since 1996. She has been published on several websites and in the winter 1996 issue of "QECE." Conor specializes in home and garden, dogs, legal, automotive and business subjects, with years of hands-on experience in these areas. She has an Associate of Science (paralegal) from Manchester Community College and studied computer science, criminology and education at University of Tampa.