Symptoms of a Bad Proportional Valveby Emily Retherford
Your car's proportional valve regulates the amount of pressure applied to the rear brakes, minimizing rear wheel lock up. The valve also helps compensate for differences in breaking conditions when you drive by elevating some of the pressure going to the rear brakes and applying it your car's front break system. Like with any car parts, problems with the proportional valve can arise. Learning how to determine if the proportional valve isn't working right could save you the expense of completely replacing your brakes.
Rear Wheel Lock Up
If the proportional valve isn't working properly, an equal amount of pressure is applied to all four wheels during a stop. Since more pressure is needed on the front brakes, this causes the rear wheels to lock up before your car comes to a complete stop. The situation is dangerous -- especially on wet roads --because it cause your car to skid before coming to a stop.
No Fluid Pressure
If a proportional valve is bad, then your car won't have the same amount of fluid pressure at all four tires. To check the fluid levels, put the vehicle up on a jack so that all tires are level. Crack the bleeder screws on all four brakes. If you don't have the same amount of pressure on each bleeder screw, the proportioning valve may be bad.
If the proportional valve isn't distributing the correct amount of pressure to the front and back breaks, it's difficult for the car to stop. If you have a bad proportional valve, your car takes longer to stop because the proper amount of fluid pressure isn't sent to the front brake system, where it's needed to come to a quick stop.