Rear disk breaks

If you ever NEED a c-clamp to move the piston, you got junk. The piston has a ring of dirt or rust that is catching on the square-cut seal and it's going to do the same thing when you release the brakes. We used to always rebuild calipers and check for that but today professionally rebuilt calipers are cheap and it doesn't pay to go through all that work yourself. If there's nothing wrong with the pistons you will be able to easily pry them in with a large flat-blade screwdriver before you unbolt the calipers.

In your case the rear caliper has the parking brake built into it. That is a miserable system that Ford and GM used for many years. They are using the the much more reliable and simple Chrysler system now on their newer vehicles. You need a special tool to screw the pistons in. You'll see two or four notches for the tool to catch and grab onto. Many auto parts stores borrow or rent tools and they will have what you need.