Road Runner Nest

Proportioning Valves. If a vehicle has been driven for a long time without rear brakes, and now you are trying to bleed them will they bleed? Here's the deal: I replaced all the lines, hoses, and the master cylinder. The fronts bleed and work fine, but the backs won't bleed. Does the prop. valve need reset or replaced? I read in my Haynes book that you should turn the key to on and push the brake pedal down slowly to re-center the piston or whatever in the prop. valve. What do you guys think? Do you think it will work?

Tough call. The valve has preset pressure relief spring to cut down the amount of pressure going to the rear brakes ina Disk/Drum set-up.. Those could be stuck or rusted. Do you have a factory manual so you can see the cut away view of the workings?

There are two pistons at work in the combo valve. One is a piston that slides side to side IF pressure is lost on one side of the system, The piston provides "ground" to complete the brakelight warning circuit.

The other piston doesn't really "center". It is under spring pressure to keep the rear brake line from seeing any pressure until the front brakes have applied. This piston/spring also regulates the total amount of pressure the rear brakes see.

So you have two possibilities.

1) The brake warning piston is slammed and stuck to one side.

2) The spring/pistion regulating combo is stuck or rusted in place