'96 A6q - spongy brake pedal feel, MC or caliper?

[Eyvind E. Spangen]

My '96 A6q has got a spongy feel to the brake pedal. It has always (for the
two months I've owned the car) been like that. If I pump the pedal, it gets
firmer and better. I was suspecting air somewhere. Today, I replaced the
rear pads/discs. Cleaned and lubed the guide pins, bled all 4 wheels and
everything seemed OK. But no.. The pedal is still spongy and will almost
bottom out. If I pump it, engine running or not, it will get better. Wait
two seconds and try again, and it's bad. No visible leaks anywhere.

One of the rear calipers did not move the piston towards the pads by pumping
the pedal, even though I'm sure all the air has been bled. I tried turning
the piston outwards and the pedal feel got better. There's plenty of
pressure in the line when pressing the pedal. Looks like the piston won't
move towards the pads, or it slips back too far from the pads. Never seen
anything like this

The caliper looks like it could be the culprit. Do you agree, or should I
suspect something else, like the MC?

-- 
MVH/MfG
Eyvind E. Spangen