I have replaced all my brake lines and two wheel cylinders. I have rebuilt the other two cylinders and the master cylinder. My brakes still do no hold pressure. After a couple pumps they hold, but then if you let off, in less than a minute the pedal will go back to the floor as soon as you step on it. I have fixed all the leaks I could find and bled the brakes several times now. Is it possible my master cylinder is bad but not showing signs of a leak? Maybe my problem isn't air in the system but just a bad part? Seems like a pretty simple system but I cant figure this out. 1950 3100 Chev.
