rear brakes on Toronado?

I have a 1988 toronado and have to replace the rear brakes but WHEN i PULLED THE REAR CALIPER OFF i COULDN`T GET PISTON TO TURN IN OR PRESS IN FAR ENOUGHT TO FIT NEW BRAKE PADS .......IT DID TURN IN A BIT ....DO i NEED TO USE SOME KIND OF PRESS TO TRY TO PUSH THEM IN FURTHER
SEEMS PISTON IS TURNED IN AS FAR AS IT WILL GO
tHANKS