Braking Components - Selection and Design
Customers often ask: "Which outlet on the master cylinder should go to the front brakes and which should go to the rear?"
Master cylinder designs vary from one model or manufacturer to another, and many have the front outlet going to the rear brakes and the rear to the front, while others have the opposite arrangement. Many dual outlet master cylinders will have a built in proportion to them. It is imperative that the installer knows what master cylinder he has and how it was intended to be plumbed.
What booster size should I use?
Power brake boosters require a minimum of 17 inches (or more) of manifold vacuum to function properly. Smaller boosters will typically require more vacuum to perform adequately. A smaller booster is a great option to consider when you’re mounting in a tight location like under the floor on the frame, or on the firewall. For full pedal assist manufacture requirements vary between 18-22”of vacuum.
When using a more radical lift cam with lower manifold vacuum supply, one option is to use a 12 volt vacuum pump available from Speedway under part number 91028146. We also offer our power brake vacuum reserve canister, part number 5503701. This will add braking performance on vehicles with marginal manifold vacuum at idle. The canister simply catches high vacuum conditions and stores it for use when the brakes are applied.
NOTE: One of the main differences between a single and dual diaphragm booster is simply that a dual diaphragm booster offers more pedal assist.
What size master cylinder should you use?
One common misconception is that a larger bore size will generate more fluid pressure. The fact remains that a larger bore master cylinder will create more displacement; however fluid pressure will increase with a smaller bore size, as a larger bore size will require more force to achieve the same pressure output.
A 7" booster like Speedway part number 91031430, will accept master cylinders designed for a power brake system. They have 3 3/8” center to center mount pattern. Speedway offers a 1” bore GM style master cylinder, typically recommended for front disc/rear drum or four-wheel-disc applications under part number 91031445. We also carry a GM style 1 1/8” bore master cylinder under part number 91031440. In addition, we carry several other options in chrome, polished aluminum, and performance versions from CPP and Wilwood.
For anyone designing their own brake system, we recommend attempting to emulate what the factory design was. Particularly, using oem type designs and equipment. From the 1940's through the 1970's, the majority of vehicles produced by Ford, GM, and Chrysler used 1" to 1 1/8" bore master cylinders on average passenger cars.
10psi and 2psi residual valve uses
The main idea here is to prevent the possibility of a squishy or unresponsive pedal caused from fluid bleed-back. Our technicians recommend installing 2 psi residual valves for a disc brake circuit any time the master cylinder is mounted below the horizontal plane of the brake caliper. Drum brakes will always require a 10psi residual valve regardless of the master cylinder mounting position. Residual valves function as a one-way check valve, with the main purpose of maintaining fluid pressure at the caliper or wheel cylinder.
It's recommended that residual valves be mounted as close to the master cylinder as possible. Because drum brakes require more pressure to over-come the spring tension on the shoes, we recommend using a 10psi residual valve like part number 91031347. A 2psi residual pressure valve like part number 91031345, is installed for the disc brake circuit.
Proportioning Valves
Having the front-to-rear brake pressure balanced is essential to better braking performance. A brake proportioning valve simply adjusts pressure on a brake circuit. Most vehicles use a little less line pressure on the rear brakes to keep them from locking up in a panic stop. The actual requirement can vary from vehicle to vehicle, as many have different tire sizes and front to rear weight distribution. Our technicians recommend installing a proportioning valve on nearly every application. Speedway Motors offers part number 91031354, a fully adjustable proportioning valve to allow adjustments in line pressures.
The diagram below shows a typical streed rod brake system with the master cylinder mounted on the frame.
