Vacuum Assist Brake Booster

A vacuum operated brake booster requires manifold vacuum to assist brake pedal effort. The three types of vacuum brake boosters are the lever, disc and tandem. A tandem brake booster has two diaphragms. Using two smaller diaphragms reduces the size and weight of the unit. Not all engines produce enough vacuum in the manifold. For these engines manufacturer's have installed electric, belt, or cam driven units to create additional vacuum.

Two smaller diaphragm's equal the same assist as one larger. This means that 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 type and the disc type are the parts used to react to the master cylinders return force. A lever type booster uses a lever for react pressure back to the pedal while the disc type uses a rubber disc. There's a mechanical connection between the pedal side of the booster and the master cylinder side. This means there is always a manual connection in case there's a loss in vacuum. This rubber disc is part of this mechanical connection and gives the brake pedal the right “feel” when applying the brakes.

Brake booster resting.

A vacuum operated brake booster has two chambers, the brake pedal side and the master cylinder side. These two sides are separated by the 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 pedal 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. These are the air and vacuum valves.

Brake booster applied.

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

Diagnosing a vacuum brake booster is fairly simple. Always do a complete visual inspection and ask the customer about the braking complaint. If the vehicle has a hard brake pedal and is having difficulty stopping suspect the booster. 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 pedal 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. When confronted with a soft pedal or a pedal that fades toward the floor suspect the master cylinder. Leaking primary and secondary seals result in a brake pedal that fades to the floorboard.