[V8] Serious brake problem

[Gunter Lorberg]

Guys...I am stumped...
The left rear brake on one of my V8's has not seized up solid, but the piston/pads are clamping down on the disc to the extent that the car almost doesn't roll when in Drive, and after driving for no more than two miles at low speed, the brake heats up so much that the rim is too hot to touch and there is production of smoke.
I am satisfied that I have ruled out the common causes.....
- replaced rear brakes in their entirety.....discs, pads, and calipers....everything new appears to functioning properly.....problem predates replacement of these parts.....it's as though replacement resulted in no change whatsoever....when installing that caliper, swap was easy.....caliper FULLY retracted....pads not clamping down on disc
- 100% certain that it is not the brake cable......cable moving smoothly....actuator arm on caliper working smoothly....I have looked at emergency brake mechanism when brake malfunctioning and super hot.....cable working well and arm NOT engaged
Other symptoms.....brake pedal very 'mushy' when engine running.....pedal can almost be pressed to the floor.....overall braking power significantly diminished......red brake light flickers on briefly when brakes depressed rapidly.  Switching ABS off (with yellow ABS light confirmation on) results in no change.  Brake master cylinder changed with NO effect.  All fluids topped up.  Braking system bled multiple times.....unremarkable findings upon bleeding....no benefit.  Hydraulic system appears to be functioning properly.  Brake hoses appear to be fine.....not ballooning.   Rear pressure regulator arm probably seized due to corrosion but car doesn't carry heavy loads--these are seriously expensive new from dealer.  Right rear brake appears to be functioning normally....does not heat up.
I am puzzled.....what is wrong with my braking system? ..... This 1990 V8 has UFO's, automatic, and about 230 000 miles.  
Gunter