How to Pick a Brake Booster


To function correctly, brake boosters require a minimum of 17 inches of manifold vacuum. Typically, smaller boosters will demand a greater vacuum to function effectively. When installed in a confined area, such as under the floor on the frame or the firewall, a smaller booster is an excellent alternative to consider. Vacuum requirements for manufacturing full pedal-assist vary between 18 and 22 inches.

How to Pick a Brake Booster

When employing a cam with a more radical lift and a lower manifold vacuum supply, one alternative is to use a 12-volt vacuum pump. This will improve the braking performance of automobiles having a marginal intake vacuum at idle. Simply, a canister stores high vacuum conditions for usage when the brakes are activated. The primary difference between a single and two diaphragm booster is that the dual-diaphragm booster provides additional pedal pressure.

A widespread misunderstanding is that a larger bore size will produce greater fluid pressure. Unchanged is the fact that a master cylinder with a larger bore will produce greater displacement; nevertheless, fluid pressure will increase with a smaller bore size, as a larger bore size will take more effort to generate the same output pressure.

A 7″ booster is compatible with power brake system master cylinders. They have a center-to-center mounting pattern of 3 3/8 inches.  A 1″ bore GM-style master cylinder, which is normally suited for front disc/rear drum or four-wheel disc systems.

Anyone developing their brake system and aims to replicate the factory design, specifically utilizing OEM designs and equipment. From the 1940s to the 1970s, the majority of Ford, GM, and Chrysler passenger automobiles had master cylinders with a bore size of 1″ to 1 1/8″.

The objective is to prevent pedals from becoming mushy or unresponsive due to fluid bleed-back. When the master cylinder is located below the horizontal plane of the brake caliper, technicians advise adding residual pressure valves of 2 psi for a disc brake circuit. Regardless of the installation position of the master cylinder, drum brakes always require a 10psi residual valve. Residual valves are one-way check valves whose primary function is to maintain fluid pressure at the caliper or wheel cylinder.

It is advised that residual valves be located as close as possible to the master cylinder. Because drum brakes require greater pressure to overcome the spring strain on the shoes, we advise utilizing a 10psi residual valve. For the disc brake circuit, a 2psi residual pressure valve is suggested.

Balanced front-to-rear brake pressure is crucial for improved braking performance. A brake proportioning valve modulates the braking circuit’s pressure. The majority of automobiles use slightly less line pressure on the back brakes to prevent them from locking up during an emergency stop. Due to the wide variety of tire sizes and front-to-rear weight distribution, the actual requirement can vary from vehicle to vehicle.