So my attempts to correct the right pull of my brakes, by changing the hose, did not work (I have to say that the old hose could have been safely stored away for another 1000 years in a cave near the Dead Sea, so it wasn't a waste of time). I also removed, cleaned and replaced the guide pins a week ago with the hose. The piston does appear to be stuck, or at least is very reluctant to move.
I replaced the rotors and pads 5 years ago (only 10,000 miles max) so do not want to damage the pads getting the caliper off.
I'll be installing an exchange unit, so reasonable levering on the caliper is acceptable. Levering against what, that is the question.
Given the rust ring around the rotor perimeter, how can I get the piston back far enough to remove the caliper? I couldn't find anything much on the subject, except for some You Tube videos of shade-tree types working on US made cars, using hefty screwdrivers. What is the technique that will also not wreck the pads or the rotor. Many thanks.

 

Hefty screwdrivers and prybars have always worked at getting the pads/piston started back into the caliper housing to slide it off.
I like BFHs for other applications when warranted.
The 'judicious application of force' is a learned and valuable tool in auto repair.

 

Open the bleed screw to help and normally it would not require much force. However, if the original boot was not installed properly and rust got in, you will not shift the piston at all. I had mine on the bench with 8 bar of air behind it and not moving. I had to put a stillson/pipe wrench on it to turn the piston before it would come out.
Best bet would be to clean off the rotor rust ring on one section and try to lever/hit the caliper off over it. You may or may not trash the pads - but they are a lot cheaper than gouging a chunk out of the rotor.