School me on how to recenter proportioning valve piston

Help,I believe my proportioning valve is out of wack. Brake warning light stays on. I have to pump up brake petal to stop and it feels as though only front disks are working when I do this. I disconnected the electrical wire to the proportioning valve and warning light goes out. I was told that this means there is something wrong with the valve itself (possibly the proportioning valve piston is not centered). What steps do you take to get the piston re-centered correctly. Brakes were fine when car was put away last fall.

1971 Demon front disks rear drums

I've never had a problem with it sticking. make sure all your brakes are bled and there is no air anywhere, then with everything sealed back up, a nice good hard stand on the brake pedal should be all it needs to "re-center"

Also, make sure your brakes are all adjusted correctly. the auto adjusters rarely auto-adjust, and at every oil change they should all be checked and evened up. could be that your rears are loose, and the pedal stroke is going to pushing the brakes out, instead of building pressure, which then triggers the piston on the warning switch to light up because the fronts are tight, and built pressure.

Do as advised.then if its stuck I have someone stab the brakes real hard as I open a frt bleeder.Ive also had to tap on a few with a hammer to get them to move while opening a bleeder.Rocky

I think you have a problem with your master cylinder.

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Brake warning light stays on. I have to pump up brake petal to stop

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(1) Bleed the M/C (2) then all 4 corners in order & get the air out (3)crack a bleeder on (prob) one of the front caliper bleeder screws as a helper stabs the pedal a bit at a time to shift the valve over (till the light goes out). when he stabs the pedal down a certain amt have him hold it there till you close the bleeder then repeat till the light goes out.