hi I HAD TRouble bleeding mine also . Ill bet if you rebuild master you will have better luck bleeding.Mine was loaded with tiny pieces of rubber. Amazon 32 bucks,,rebuild kit.

 

I think this is the issue, thanks for your help. Looking in the reservoir there are small "brown globs". I gotta imagine this bike sat for a while with little maintenance. I'm going to rebuild for sure. I don't think the rear brake ever worked that well to begin with since I've had the bike.

Here's a shot of the reservoir. Didn't see this when I put a new brake line on the front brake.