Vacuum Assist Brake Booster

Vacuum brake booster and brake master cylinder.

A vacuum-operated brake booster requires 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 smaller diaphragms that reduce the size and weight of the unit. Two smaller diaphragms equal the same assist as one larger diaphragm does. Two 10 inch diaphragms apply the same force as one 20 inch diaphragm while reducing the unit's size and weight.

Not all engines produce enough vacuum in the manifold. For these engines manufacturer's have installed electric, belt, or cam-driven pump units to create the vacuum.

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 is in case there's a loss in its vacuum supply. 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 separated by a diaphragm. A constant supply of vacuum is controlled 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 pressure) into the pedal side of the booster assembly while maintaining a vacuum on the master cylinder side. This vacuum helps pull (assists) the power piston towards the master cylinder, significantly reducing the amount of effort required to apply the brakes. It is accomplished using two valves located on the peddle side of the booster. These are the air and vacuum valves.

Brake booster applied.

The brake pedal controls these valves. The air valve allows air to enter the chamber while simultaneously the vacuum valve cuts the vacuum off. The two halves once shared this vacuum. Blocking vacuum on the pedal side, while allowing atmospheric pressure into the chamber, assists the diaphragm towards the master cylinder. This action dramatically reduces pedal effort.

Diagnosing a Vacuum Brake Booster

Always do a complete visual inspection and ask the customer about the braking complaint. Check if the vehicle has a hard brake pedal and is having difficulty stopping; if so suspect the brake 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 towards the floor a specified amount (an inch or two). Always check the vacuum supply and compare it with the manufacturer's specifications.

Leaking Master Cylinder Cup Seals

Leaking primary and secondary  master cylinder cup seals.

On the other end of the spectrum is the master cylinder. When confronted with a soft brake pedal or a pedal that fades toward the floor, suspect the master cylinder. Leaking primary and secondary master cylinder cup seals result in a soft and fading brake pedal.