Proportion valve/ brake troubles

[Mr.Smurf]

Hey guys, long time viewer. first time poster. I have an 83 GMC 1/2 ton short box 2wd(originally 305/350 combo, now w/ 350)  I drive it occasionally but it mainly stays park as I continue to work on it. Now the brake warning light constantly stays on. The light goes out if I unplug the proportional valve wire. Also the back brakes are locked up as though the parking brake is on(even tho its not). I did do a brake job over the summer. Replaced all 4 pads, rear drum cylinders, and bleed all brakes. Haven't had any issues until now. Does the proportional valve need replacing? or is there a bigger issue with the rear brakes? Also where is a good place to find a new valve? my local part stores don't carry them

[burbboysgc]

you will have to get the prop. valve new at the dealer or try your luck with a salvage yard part. Im having same problem with my burb.

[VileZambonie]

It's called a combination valve. The differential pressure valve inside grounds the light switch when there's a loss of fluid pressure on either side of the system. Bleed the system and pump the pedal firmly.