94 Saturn SC2 -- Rear brake caliper problem

94 Saturn SC2
AT - 4 wheel disc brakes
161,000 miles

I am replacing the rear brake pads and rotors. When I attempt to push the caliper plunger in, it will not budge. This is the same on both sides. I am using a large c-clamp which I have used to do other brake jobs and have not run into such a problem. Is there some reason why I cannot get the caliper plunger to push in?

Thanks.