A proportioning valve must be used with cars that use front disc and rear drum brakes. The proportioning valve is installed in the rear brake line and reduces hydraulic pressure to the rear wheels to help reduce rear wheel lock up under severe braking. The rear wheels will lock up first because under heavy braking most of the vehicles weight is transferred to the front.

A combination valve works the same as a proportioning valve but combines it with a low brake pressure warning light and metering valve. A combination valve also acts as a junction block for the two master cylinder lines and the lines to each wheel. A special valve is used for disc/disc systems.

Also called a "hold-off valve", it is used on some vehicles to balance front disc and rear drum brakes. It slows the application of the front discs to allow for the slower reaction of the rear shoes so that the front pads and rear shoes contact the rotors and drums at approximately the same time.

Pedal ratio is determined by dividing the distance between the pedal pad and pivot point and the pivot point and master cylinder push rod pivot. Typical pedal ratio for manual brakes is 6 or 7 to 1 and 4 or 5 to 1 for power brakes.

Residual pressure valves are used when the master cylinder is lower than the calipers/wheel cylinders such as in an under the floor installation on a really low street rod. This situation is made worse if the car is parked on a hill. The valve keeps the fluid from draining back into the master cylinder. 2 psi valves are used on disc brakes only. 10 psi valves are used with drum brakes to maintain a slight pressure on the wheel cylinders. This keeps the piston seals in contact with the cylinder bore when the brakes are off and keeps air from entering the system maintaining a firm pedal.

Master Power Brakes offers a 7", 9" and OEM size boosters for some applications. In general, use the largest size booster that will fit. In some cases, such as a street rod with the master cylinder under the floor, you may have to use the 7" booster which is designed for use on a lightweight car. It is specially valved to not "overboost" or be too touchy on a light car. If you have a heavier car or a big cam radical engine, use the bigger 9" booster It has a larger capacity to store more vacuum and a larger diaphragm. We also have a dual diaphragm "maxi booster' designed for big blocks and four wheel discs.

A booster works by applying engine manifold vacuum to a diaphragm in a sealed housing. When the brakes are off, vacuum is supplied to both sides of the diaphragm equalizing it. When you step on the brakes, the pedal push rod activates a valve that lets atmospheric pressure into the rear chamber of the booster. The positive atmospheric pressure acts against the diaphragm and the return spring applying pressure to the master cylinder. The vacuum "pulling" against the diaphragm helps apply the brakes. The calibration of the valve and return spring prevent the pedal from being too "touchy" making it difficult to apply the brakes smoothly.

Yes, because the rear calipers have a larger capacity than wheel cylinders, the master cylinder must compensate by having a longer stroke. And ours feature special anodized hard coating pistons and use only American made castings, rubber and pistons.

1. Using the wrong pedal ratio.

2. Incorrect push rod adjustment. There should be about .015" clearance between the booster push rod and the master cylinder piston.