I can imagine that you are being desperate but well anyway here it comes: Have you cleaned the pistons of the rear brake. Once they stuck they are dry, so if the pistons' surface is not lubricated they have a tendency to stuck.
How to repair: Pull the whole assembly take of the brake pads and push the pistons out one at the time. Take a socket or nut to block the other. Once out. Be carefull not to push them out completly clean them and use fine sand paper (1000 to 2000 grain) and polish the pistons. Use good grease to lubricate them and do the same on the other. All in all about 30 minutes to one hour.
I keep a old tooth brush in a grease can and use the it on the pistons and the chain drive.