88 GMC C2500 Brake problems

Hi have a 1988 gmc c2500 with brake problems. the front brakes keep locking up both sides. Have replaced the calipers rotors pads and the brake lines. I can bleed them off and they release but has soon has you it the brakes they loock up again. Im at wits end any help would be great thanks.

Brake hoses were replaced, no? Any work done on the master or booster, then this happened? Do you have Hydro-Boost.

Yes lines were replaced nothing done to the master or booster

no hydro boost

Ok, when the brakes lock up, disconnect the vacuum hose to the booster see if they free up.

Hi tried the line going to the booster they still stay lock up after taking off the air line

Are you sure the hoses were replaced you keep saying lines? Also make sure there is at least 1/8-1/4 free travel in the brake pedal.

yes hoses were replaced and i have 1/8 to 1/4 travel in the pedal.

Possible the proportioning valve is bad and holding pressure, not much else left.