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The proportioning valve simply ensures that the front brakes are applied before the rear brakes are, to prevent sideways skids from loss of rear traction. The valve also serves as a warning light when one end, either front or rear, of the system has deficient line pressure. Inside the valve is a cylinder, and a piston that belongs in the center of the valve cylinder (when fluid pressure on each side is equal). When one side of the valve (front or rear brake system) loses pressure (like from a leaking brake line), the piston gets pushed to the low pressure end and makes contact with a switch terminal that turns the light on.The trick to turning the light off is to figure out which end, front or rear brakes, the piston is on, and then bleed the other end to center the piston and turn the light off. Too much pressure on the brake pedal will cause the piston to go past center to the opposite end, and a close watch on the brake warning light will reveal that as the lamp will flicker once while the piston passes center. Gently depress the brake pedal (requires an assistant) with a bleeder open and close the bleeder as soon as the assistant (brake pumper) sees the light go out. If he sees the light flicker once bleed the other end of the vehicle to go back towards center.
Try pumping the brakes...this is necessary after re-positioning the brake pistons...You either have a brake line leak...or your master cylinder is bad...try bleeding each side to remove any trapped air...be sure to recheck brake fluid level after bleeding...you can bleed at the brake line connection to each side...on each rear caliper...Hope this helps.
If you have disk brakes, and you cannot fit the new brake pads in the callipers? Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front and back of the piston. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.
It could be a proportioning valve, and there's also the chance that the portion of the master cylinder that operates the front brakes is going bad. Have you had a pressure check done on the front brake system? Also, some vehicles have a variable proportioning valve, so that when the vehicle is loaded, it allows more pressure to the rear brakes. These can sometimes stick in place, causing the fault that you describe. Hope this helps you out.
You have to turn (rotate) the piston to get it to go back. A c-clamp will not work and it may damage the piston. I have a 1995 and I just replaced the pads on the rear this past summer. I did what you did and tried the c-clamp and it didn't work. I then read up on it and you have to turn the piston. I did this with some needle-nosed pliers and put them into the valleys on the piston to turn it. I can't remember which direction I turned the piston. I think it was clockwise, but I am not sure.