[panhandler62]

I'm not personally familiar with those parts but; is there an inside and outside to the pads? Once upon a time I put the pads on backwards on an old Mercury beater I had and it acted like that -- all was well until I hit the brakes then they would clamp down and stay that way.

If that's not it (or not possible with those calipers, etc.) then...

This is just off the top of my head but; if this isn't a common symptom that someone recognizes I would start by splitting the problem.

Step on the brakes a few times till the rears lock then take a fitting off and see if they free up. (this will be messy , of course) If they instantly free up (I'm guessing you have an idea how long they stay clamped until they free on their own now) by pulling the fitting at the proportioning valve then the valve is your issue.

If they don't then it is probably in the line, somewhere. I am assuming the symptom is the same for each rear wheel here.