Have tried using a g-clamp, but it will only move 2-3mm, had no trouble with the front brakes.
Ans: Push in while turning the piston clockwise there are tools that put pressure on the piston while turning notice the two spots on the caliper piston face there the tool pins catch turn and push inward. Your rotor surfaces have been trued with a lathe? If you put new pads on old warped rotor surfaces you did not do a very good brake job. And do you know those shims on the backs of the old pads are there for a reason? Brakes should be worked on by persons with years of experence with braking systems so they operate quietly and stop in the shortest distance.
I had a similar problem on a Honda. It was the parking brake set up. Look for the parking brake cable and see if it is on a ratcheting mechanism. I can’t tell you how to release it but if you are working on the brakes you must know something about your car. Just look it over and see if you can break the code. If I remember the Honda had a pawl on it that I had to hold back as I unscrewed what amounted to an auger that kept a sort of preload on the piston so the parking brake can self adjust. If you are familiar with self adjusters on drum brakes the Honda worked in a somewhat similar manor only it screwed into the piston from directly in line with the direction the piston pushed.
Good luck. And don’t break it with the c-clamp, I almost did and it would have cost me many $$$$ to fix it for the customer and the money would have come from my pocket. Email my profile if I can help any further. I’m not sure my answer is all that clear.