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Vacuum Assist Brake Booster

A vacuum operated brake booster typically uses a port on the intake manifold as a source of vacuum. Diesel engines don’t produce the vacuum necessary to operate these boosters so an electric, belt, or cam driven unit is used. There are three types of vacuum brake boosters: the lever and disc type both have a single diaphragm. The tandem vacuum brake booster has two diaphragms. They use two smaller diaphragms to reduce the size and weight of the unit.

The engineers found that two smaller diaphragms equal the same assist as one larger unit. Two 10 inch diaphragms apply the same force as one 20 inch diaphragm while reducing the units size and weight. The only difference between the lever and disc type are the parts used to react to the master cylinders return force. The lever type uses a lever to react pressure back to the peddle while the disc type a rubber disc. There is a mechanical connection between the peddle side of the booster and the master cylinder side. This rubber disc is part of this mechanical connection and gives the brake peddle the right “feel” when applying the brakes.

A vacuum operated brake booster has two chambers, the brake peddle side and the master cylinder side, separated by a diaphragm. A constant supply of vacuum is regulated by a vacuum check valve located on the master cylinder side of the booster. A vacuum operated brake booster works by allowing atmospheric pressure (outside air) into the peddle side of the booster assembly while maintaining vacuum on the master cylinder side. This vacuum helps pull (assists) the power piston towards the master cylinder greatly reducing the amount of effort required to apply the brakes. This is accomplished using two valves located on the peddle side of the booster: the air and vacuum valves.

These valves are operated by moving the brake peddle. The air valve allows air to enter the chamber while the vacuum valve cuts the vacuum off. This vacuum was once shared by the two halves. The action of blocking off the vacuum while allowing atmospheric pressure in lets the diaphragm move towards the side still under vacuum (master cylinder side) assisting in braking.

Diagnosing a brake booster is usually straight forward. Always do a complete visual inspection and ask the customer about his braking complaint. If the vehicle has a hard brake peddle and is having difficulty stopping turn off the engine and pump the peddle 20-25 times (check the manual if unsure) and then start the engine. If adjusted correctly the peddle should move down towards the floor a specified amount (an inch or two). Always check the vacuum supply against manufacturer specifications.

On the other end of the spectrum is the master cylinder. Leaking primary and secondary seals leave a peddle that fades to the floor board. If the rear drum brakes are adjusted correctly and the peddle feels rock hard there is likely a problem with its power brake booster.