Brandon240sx

After my s13 was sitting for over a year I decided to do brakes before driving it. Front install went smoothly. Now I’m at the rear, I have one side back on and together but the other side I cannot get. I rented the tool from autozone to twist the piston back in but on one of my rear calipers I can’t get the thing to budge and twist in. The caliper is a little rusty but not terrible. Im guessing my caliper seized up? Any one have anything I can try to get this thing in? Tried putting some pb on it which didn’t help much.