I think
rhandwor is right with the collapsed brake hose. I also just did my rear brakes, but the pass side was the issue. We thought it was the parking brake because I use it a lot and the cable had cracked and rusted through. After replacing the cable, it started doing it again. When we were bleeding the brakes, I would pump up the peddle and the drum would not move. Open the bleeder and close it, it was fine. Replaced the hose and I am driving good as new. You really can't shut it off.
Good Luck and hope you fixed it.