After replacing all 4 flexible brake lines I was unable to successfully bleed the system. Brakes worked fine before I started. Replaced the master cylinder (turned out it was needed by looking at it). Bled the system and got a good pedal. Left the car for a while. Came back to find pedal nearly to the floor. There is no evidence of any leakage. Any ideas out there?

did you do the bleeding alone?

I bet it has somthing to do with the proportioning (sp?) valve. On other cars I have run into this problem before, once even going through a 4 liter jug of fluid! I finally gave up and cracked the lines as I blead. Guess what? IT WORKED. I don't recommend this method but 'what the hey'! It was messy though.