'83 Olds Toronado brake problems

I can't seem to get my rear brakes to bleed out without something going wrong with the driver's side rear caliper. It was the original problem with my system when it started leaking near the e-brake arm. The replacement (rebuild) did the same thing, as did the next, and the next, and the next. After looking for problems elsewhere, I've replaced the master cylinder, proportionary valve, and a few lines, but I'm still having troubles with this caliper. Either the emergency brake will engage on its own, or the piston won't retract. Pressure appears to be even in both rear calipers, and higher in the front than the back. No troubles with the other calipers, they fully engage and retract.

Is there something I'm missing?

are the pads wearing unevenly between the two rear calipers?

Might have a blockage at the proportioning valve, or at the Y block (whichever is applicable) causing pressure to not release rapidly at that caliper, but you would have uneven pad wear if that was the case, along with other issues

Pad wear looks even on both sides.

You should open the proportioning valve plunger, mounted up under the master usually on the frame, there is a button on the end of it that should be pulled out about 1/8" to allow fluid to the rear.