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Proportioning Valves

The typical plumbing situation for a proportioning valve is to run full master cylinder pressure to the front brakes whether they are disc or drum. Today, most cars have disc brakes up front. The brake line to the back brakes usually has the proportioning valve plumbed in between the rear brakes and the master cylinder. Most hot rodders will run an adjustable proportioning valve to set the optimum brake pressure setting rate. Custom built cars with aftermarket brakes can be hard to tell what you should set things at. Proportioning valves help to customize brake pressure rate settings to the rear wheels.

Here is where the confusion comes in. Proportioning valves don't limit the pressure all of the time, just the rate at which the pressure is applied. Picture this: You step on the brake pedal, full pressure from the master cylinder forces brake fluid to the front brakes. The master cylinder pushes fluid through the back brake line to the proportioning valve at the same rate as the front. The proportioning valve limits the amount of fluid that can go through it. This lowers the pressure rise to the rear wheels. Effectively less brake fluid is passing through the proportioning valve when compared to the front brake lines. Eventually (we're talking seconds or less), full pressure is applied to the rear wheels, just as the front. The pressure equalizes between front and rear. With a proportioning valve in place, the front brakes get full pressure first, the back brakes get some pressure at first and it rises to full pressure over the braking sequence.

Vehicles with more weight on the nose tend to need proportioning valves more. Without a proportioning valve, you would push the brake pedal and the rear wheels would lock up. If a car or truck is nose heavy, the weight pushing down on each end of the vehicle is less. That lowers friction between the tire and road at the rear of the vehicle. Thus you get rear wheel lockup. Also when you hit the brakes, most cars will nose dive to some degree, transferring more weight on the nose of the car. This increases friction on the front, but takes weight off the rear. That reduces friction between the rear tires and road even more and it makes rear brake lockup worse. One helpful solution is to balance your vehicle for weight as much as you can. More importantly is to set your brakes up with a proportioning valve. Now with a proportioning valve in place. You hit the brakes and full pressure is put on the front brakes. Less brake pressure is applied initially to the rear brakes, counteracting the weight transfer/reduces friction issue I talked about above. Then as the weight transfers back to the rear over our braking progression, the weight returns to the back of the vehicle adding weight over the rear wheels. Full brake pressure is rising through this progression. It can take some time to get a proportioning valve set up right, but it is important for safety and proper brake performance.