I'd assume the piston is stuffed anyway. Just replace it, along with replacing all the caliper seals. I'd go ahead and throw a rebuild kit in the master cylinder also, and replace the brake line.
Damn man you ain't scared to spend somebody's money are you?????
I just got this 1995 Heritage Softail and I'm replacing the completely worn out rear pads, but cannot get the brake caliper piston to compress. I've removed the front reservoir cover, opened the rear caliper bleeder valve and with using a screw clamp, still can't get the piston to budge. The caliper bleeds OK when tested. I'm about to try removing the brake line, opening the bleeder valve and try again. Any suggestions? Is it possible the piston is frozen and the caliper needs to be rebuilt? I've never encountered a situation like this where the piston won't compress. The rear brake worked OK on the 50 mile ride home and didn't drag and it bleeds OK, but I'm stumped.
I am not sure if this was just improperly stated or if that is what you actually did. If you are working on the rear brakes, you shouldn't have to mess with the front master cylinder. Did you take the cover off the rear master cylinder or the front one?
Damn man you ain't scared to spend somebody's money are you?????
Damn straight!
But seriously, on a bike that's that old and was sitting for that long, and on something as important as the brakes, and given that you can probably buy all the parts you need for under $100, and given that the bike is already "down"...hell yes I'd just do it all. Last thing I want to do is half-*** the job only to have to take it back apart later because I didn't whole-*** it in the first place.
Speaking of spending money, it is possible that a frozen piston caused the pads to be so worn. That would have caused a lot of heat, so check out the rotor. Look for warpage and/or cracks. Good luck.
Prot makes a good point. OP the rear master cylinder is down by the rear brake pedal mounted on the frame. I know could just be terminology but if it's bleeding it should be moving. You say you are trying to compress it try some PB Blaster on it and let it sit with some pressure from the clamp on it. Definitely sounds like you will need to do a complete tear down on it if it is stuck.
Pull it all the way off the bike so you can work on it without the hose attached on your bench.