Problem Replacing Rear Brake Pads

[Hal97Tbird]

When replacing the rear brake pads, because the new pads have so much more thickness due to new pad material, it's necessary to push the caliper piston back into the caliper housing. On the front caliper pads, you use a C-clamp to depress the piston----piece of cake. On the rear calipers, you're suppose to buy a special tool, (I did), screw the piston clockwise to get it to withdraw into the caliper body. But...the piston would only retract slightly, not enough to install the new pads. What's the secret? Since the real caliper has a parking brake mechanism built into it, I imagine something needs to be done with this mechanism to allow the piston to move all the way back into the caliper body.

(Just trying to learn how to do some simple things with my 97 TBird. Labor rates are $80 - $100 an hour here. Also, the dealer's price for new parts is twice what I can buy quality parts on the internet.)

Thanks