Check the obvious, ie there is no air in that circuit etc but it is common for the rear caliper piston to seize up on these unfortunately, and i do not belive you can buy replacment pistons so its new caliper time. Although i have been told that Hi Spec maybe able to make you a custom piston. Its a strip down and re-build what ever way you look at it. I had the opposite, my rear caliper on one side was stuck half on, mine turned out to be a sticky handbrake cable but it cost me a set of pads and disks