2003 Hyundai Tiburon changing disc brake pads

I am in the process of changing my rear disc brake pads. When i try to compress the piston to provide additional clearance for the new, thicker pads, I am unable to move the piston. I have used a C clamp. I even unscrewed the bled valve to release any pressure inside the piston. I can't move either piston in the rear. Is there something I am doing wrong?