A proportional valve is a vital component of all automobiles and trucks, allowing them to stop properly. When a vehicle is in motion, it generates momentum, which must be overcome if a vehicle is to stop. When braking suddenly, the weight of a vehicle is thrown forward. However, the ability of a tire to retain traction on the road is dependent on the weight pushing that tire down. Because more weight is on the front tires than the rear tires, if equal braking pressure were sent to the rear tires through the hydraulic brake lines, the rear wheels would lock up and steering control would be lost before the car could stop. Proportional valves are made to send more hydraulic fluid pressure to the front tires to prevent this from happening.
Proportional valves are cylindrical in shape and hollow, made from steel or cast iron. One sits between the master braking cylinder output and the hydraulic lines connecting to a vehicle's rear brakes. The other sits between the master braking cylinder output and front brakes. The output from the master cylinder connects through a port on the side, while the output port to the brake lines is covered by a powerful spring driven piston. On average, the proportional valve connecting to the rear brakes has a hollow space roughly 70 percent larger than the front brakes.
When the brake pedal is depressed, the master cylinder forces out brake fluid into both proportional valves. It incorporates a pressure increase so that, for example, if the driver were to exert 10 pounds of force on the brake pedal, the hydraulic fluid would have 500 pounds of pressure behind it. When the hydraulic fluid reaches each proportional valve, it possesses half that pressure, divided evenly between both. However, more hydraulic fluid must be sent to the proportional valve on the rear brakes to generate enough force to push up the piston and flow to the brake apparatus. Because more fluid is sent to the rear brakes to achieve this, the pressure exerted upon the rear brakes is consequently lower than that of the front brakes. Without proportional valves, 250 pounds of pressure would be sent to both front and rear brakes, causing the rear brakes to lock up. With proportional valves, 150 pounds of pressure is sent to the rear brakes and 350 pounds of pressure is sent to the front brakes.
