Brake-proportioning valves equalize the pressure from the master cylinder so the front and rear brakes apply evenly. Without the proportioning valves, the front brakes receive more braking pressure than the rear, causing the rear of the vehicle to lift and pressing down the front of the vehicle.
Automobile brakes are hydraulic systems. Pressing the brake pedal applies pressure to a plunger in the master cylinder that forces brake fluid through metal tubes to the four slave cylinders. The pressure expands the slave cylinder to force the brake pads against the disk or drum, creating friction to stop the vehicle.
Most vehicles have a proportioning valve located near the master cylinder and near each of the two rear slave cylinders. These valves operate based on the force the driver places on the brake pedal. They reduce the rear-brake pressure more in a short stop than in a long one. Without these valves, the pressure the driver places on the brake system when attempting to stop quickly is more likely to cause the wheels to lock and tires to skid. The skid reduces the friction of the tires against the road, making stopping more difficult. On vehicles with anti-lock brake systems, the manufacturer replaces the proportioning valve with an electronic system, operated by the onboard computer to control the amount of fluid the master cylinder sends to the individual brake.