Rear brakes question
 

I have a C5 2003 A6, 4.2, seems like the rear calipers are pretty tight (both), and generating some heat. I had the same trouble with a 2000 A6 I had a few years ago. What is the trick to getting these free, with enough gap, to allow the rotors to move freely? I did adjust the piston in, but seems like it gets tight again right away.

Why do I think they are too tight?

Difficult to move on a level/flat surface (pushing by hand/leg power)
Excessive heat at rotors after a few minutes of driving
Difficult to move rotors by hand with wheels removed

Any suggestions would be helpful, thank you