I changed brakes over today, and left put the pads in then pressed them as hard as I could untill it returned back to the original position. It was very stiff and took a while to do. I dont know if putting the caliper up high helped, but it was put up on the suspension while i cleaned the disc up and stuff, they seemed easier after this, but it might have just been my mind.
Take the cap off the resivoir too before doing this.
Failing that, try what mork says.
Also, the hand break is off yeah? You might need to slacken the cable though I'm not too sure as I dont have rear discs.
JAmes