Get the rear wheel on a stand. Have someone press and release the rear brake pedal. Try rotating the rear wheel. If there is a drag on the rear wheel, break loose the bleeder screw WITH the dust boot on the screw. This allows for pressure release without introducing air into the hydraulic system. If it (the rear wheels) loosens and rotates freely the problem is NOT the caliper. It could be the hose or master cylinder. If it remains tight, the caliper is at fault.
I do not know what parts such as rebuild kits are available. You can watch eBay for parts too.