Mine did the same thing. Had it fixed by a good mecho nic. He said the rear drum brakes were always a bit compressed which made it wear and lose brake pressure. The master cylinder was what I thhough initially as well but turns out that's not the issue. My car had always had a weird gurgling noise in the *** end when I turned it off which I assumed was an aor bubble, it was which was also a brake related issue.

 

Make sure your rear brake lines haven't collapsed internally. You won't see it from the outside, but inside they might be trashed. It happened to me. I had to replace both lines and cylinders. Then I found that the E brake cable was too rusted to move freely. Good luck.