I have a 2009 ford fusion. one of the callapers on the rear brake will not open. I have a "C" clamp on it trying to compress the piston. It will not move at all. What should I check?

Open bleeder valve & push caliper piston, in as
you rotate piston to screw in parking brake

Do this by hand,only use a clamp on the front calipers

Open the bleeder to compress caliper piston

If you use a c-clamp, you will ruin the parking
brake in caliper

The piston gets turned or screwed in with the correct tool,
as you also push in

You will have to use a c-clamp or brake piston compressing tool to get that piston back in, while you have the brake master cylinder fluid reservoir cap off.

easy either buy the special tool or wind them in with a pair of water pump pliers and a G clamp ,having assistant helps if they apply light pressure to G clamp whilst you wind piston in ,clockwise

Rear brake pads last about 70,000 miles,that is 5 years or more !!!

Turn the parking brake in,turn the caliper piston in first,then compress piston.

Ratchet park brake up ,after back together and system bleed (all 4 corners)

If you have disk brakes, and you cannot fit the new brake pads in the callipers? Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front and back of the piston. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.

Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front of the piston and back of the housing. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.

If it don't press in with a c-clamp fairly easily it is seized and needs replaced.

First, take a look at the piston, if it's flat or cup shaped, put an old pad at the piston and use channel locks to squeeze it back in, opening the tool wide to get it to catch the back of the caliper.
If....the piston has what looks like an "X" or cut outs, this is for a tool that spins the piston in with a special tool. (Subaru's and some other cars use this)
After you are all done and pump up the brakes, re-check the brake fluid level.

Most Fords I've worked on have a screw-in rear caliper design. Look at the caliper piston - do you see two notches on the face, across from each other? Those are the keyholes for screwing the piston back in. AutoZone rents/loans the piston retractor tool (leave them the full value of the kit, about $40, take it home and use it, then return it and get 100% of your money back). Simply fit the proper sized key disc onto the tool and screw it back into the caliper, and you're done.