Rear brake locked up, 1979 XS Eleven Special

My brother brought his 1979 Yamaha XS Eleven Special over to my house for me to work on the carbs. Aren't I lucky. :S

I noticed when he was pushing it in the garage he was having a hard time with it. When I went to move it yesterday I could barely push it around.

I put it on the center stand and sure enough the rear tire is very hard to move. Checked the brake pedal and there was no free play, so thought I had found the problem. I adjusted it to have way more then enough free play and no change in the rear brake.

So I opened the bleeder valve and nothing came out, I was expecting fluid to be under pressure, blocked port or something not allowing fluid to return. Nope, still locked up.

What could be causing this? I've never run into this before. My thoughts are pads too thick, not likely...piston stuck, either the caliper piston or the one in the master cylinder body itself.

The brake is not locked up solid, I can still turn the wheel by hand but it takes a lot of effort. The brake is also working because they will lock up solid if the pedal is pressed. Just seems strange it wont release.

I don't have a manual for this bike. Been telling my brother for months to order one, but nope.

Without any specific knowledge about that particular caliper I'll speculate that whatever brake fluid had been in there dried out and caused the caliper to stick. I have seen this scenario at least 6 times and always on bikes that have set for some time. It's probably time for a rebuild of the caliper and master cylinder along with a fresh brake hose. I bet you find lots of crusty stuff around the rubber seals in that caliper.