Ding, ding, we have a winner! Elevating the caliper (with bleed nozzle uppermost) made all the difference in the world. Two bleeds and I feel pressure, and it's like a "normal" bleed thereafter. I have it sitting with pedal tied down, I'll leave it like that for the day, bleed, do it again overnight, and I predict I will have normal braking tomorrow.
Florabunda, give it a try!
Thanks for all your help folks.
We're getting there then. If the master cylinder was bad, usually fluid would get past the piston and slow down the brake lever when it's trying to return.
A stuck or sticking caliper would resist and you'd feel pressure at the point of friction once you bled the air out. (Ever have one freeze? The pads won't retract and continue to rub the rotor after braking. The fluid heats up and expands, eventually locking the wheel and making the lever as firm as a footpeg.)
I'd try what tghsmith is recommending. Air trapped in the caliper between the bleed screw and the piston would let fluid out the bleed screw all day, and then just compress when you apply the brake without really stopping the rear wheel.
To Quote Rosanne Rosannadanna, "Nevermind".
I was writing while you were fixing.
Thanks Bob. I seem to be on the right path now. Wouldn't have believed how profound a difference this made if I hadn't tried it.
Well, better late than wrong, as I was with my Gilda Radner reference.
Emily Latella said "Nevermind", Rosanne always said "It just goes to show".
Glad you're back on the road!
Thanks again all for help - I am indeed back on the road, with fully functional brakes.