Replacing brake pads on a 2001 V6 Accord

[solanog]

Hi I replaced the front brake pads on the V6 Accord with no issues. But when I went to the rear wheels I was not able to compress the cross faced caliper piston. I even used a tool for compressing (wasn't needed in the front calipers) it but couldn't do it. The piston doesn't seem to be stuck or rusted since it can rotate with little effort but it was impossible to compress, I even loose the bleeding screw a little but it compressed just a little, only a few drops of brake fluid came out ot the bleeding screw, I also unhook the hand brake cable linkage.
Is there something I need to do beside trying to compress it? Some special procedure different from the front brakes needs to be done?
Thanks for any help you can provide.