Ok so I bought a 1991 Jeep Cherokee Briarwood edition. The bendix burned up immediately so after much research i stripped it out. I replaced it with conventional brakes. The problem is the fronts brakes build pressure but wont release unless I bleed one of the calipers off. Here is a list of parts replaced:

new master cylinder
new brake booster
replaced proportioning valve with one from wrecking yard.
new brake lines hard and soft to drivers side front
new brake calipers driver side and passenger

I initially replaced the ABS system with conventional brake booster and new master cylinder. Went for a test drive after bleeding the brakes, after a few pumps on the brake pedal it built pressure on the drivers side caliper. so i replaced the calipers thinking they were bad.

replaced calipers and bled brakes.
went for test drive and after a few pumps it did the same thing. seized up on drivers side only.
i disconnected the drivers side brake line from the proportioning valve and put a plug in the proportioning valve.
test drove it and after a few pumps on brake pedal the front passenger side brakes locked up. bled it off and came home thinking it was the proportioning valve.
went to the wrecking yard and got another proportioning valve. i put it on and it seemed to do fine. i drove it to work and my wife drove it to her parents house twice. no problems aside from a somewhat spongy brake pedal which i was going to address later with a brake fluid flush amongst other things of troubleshooting.
i thought the caliper seizing issue was over.
then last night my wife drove it to town. she went to three different stores.
as she was leaving the store she noticed the pedal was stiff. she drove it a little ways then called me. she waited for me and i came to her and bled off the pressure. then she drove it home. i cant see what the problem is.

sorry this is so long but this troubling me. i tried to fill you all the way in so that hopefully some of you cherokee geniuses can help me.

please help!