<P> A proportioning valve may be used to reduce the pressure to the rear brakes under heavy braking . This limits the rear braking to reduce the chances of locking up the rear brakes, and greatly lessens the chances of a spin . </P> <P> The vacuum booster or vacuum servo is used in most modern hydraulic brake systems which contain four wheels . The vacuum booster is attached between the master cylinder and the brake pedal and multiplies the braking force applied by the driver . These units consist of a hollow housing with a movable rubber diaphragm across the center, creating two chambers . When attached to the low - pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered . The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed . A return spring keeps the diaphragm in the starting position until the brake pedal is applied . When the brake pedal is applied, the movement opens an air valve which lets in atmospheric pressure air to one chamber of the booster . Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure . This force, in addition to the driver's foot force, pushes on the master cylinder piston . A relatively small diameter booster unit is required; for a very conservative 50% manifold vacuum, an assisting force of about 1500 N (200n) is produced by a 20 cm diaphragm with an area of 0.03 square meters . The diaphragm will stop moving when the forces on both sides of the chamber reach equilibrium . This can be caused by either the air valve closing (due to the pedal apply stopping) or if "run out" is reached . Run out occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated by the now stagnant differential pressure . After the run out point is reached, only the driver's foot force can be used to further apply the master cylinder piston . </P> <P> The fluid pressure from the master cylinder travels through a pair of steel brake tubes to a pressure differential valve, sometimes referred to as a "brake failure valve", which performs two functions: it equalizes pressure between the two systems, and it provides a warning if one system loses pressure . The pressure differential valve has two chambers (to which the hydraulic lines attach) with a piston between them . When the pressure in either line is balanced, the piston does not move . If the pressure on one side is lost, the pressure from the other side moves the piston . When the piston makes contact with a simple electrical probe in the center of the unit, a circuit is completed, and the operator is warned of a failure in the brake system . </P> <P> From the pressure differential valve, brake tubing carries the pressure to the brake units at the wheels . Since the wheels do not maintain a fixed relation to the automobile, it is necessary to use hydraulic brake hose from the end of the steel line at the vehicle frame to the caliper at the wheel . Allowing steel brake tubing to flex invites metal fatigue and, ultimately, brake failure . A common upgrade is to replace the standard rubber hoses with a set which are externally reinforced with braided stainless - steel wires . The braided wires have negligible expansion under pressure and can give a firmer feel to the brake pedal with less pedal travel for a given braking effort . </P>

How much pressure in a car brake system