Check to make sure your rotor isn't bent, and your pads are installed correctly. Also, you may want to check your rear master cylinder. If the cartridge in it is bad, that is often times the effect it will produce. If your master cylinder is good, you may have a bad caliper piston. I have never really heard of them going bad, but if a piston bore was out of round or scratched, it could cause the pad to drag on the rotor.