Other Rear brake problems

when I bought the bike, the rear brake didn't work from the start. First, the master cylinder was locked up and wouldn't move at all. I found one off of a kx100 (same as the one on my kx80) and replaced it with that one. The master cylinder works great and will pump fluid out of the top as long as the line is off. Somehow though when the line is on and connected to the rear brake, it won't pump brake fluid out when I tried to flush the brake fluid. The only thing that comes out is air! Anyone have any ideas?
 
Top