I had an issue once trying to bleed the rear but the proportioning valve was stuck and it would not move. I ended up curing the problem with anger, I got in the car put it in reverse , back up quick and slammed the brakes and after a few times l I heard the valve click and was able to bleed the brakes. I have never found out why that worked and I only did it out of frustration with working way too long on a simple hydraulic system......
