Working on a 26 Chrysler.  New front end with disc brakes.  New master cylinder and pressure regulator.  Rear end is original Chrysler and the wheel cylinders have been rebuilt.  The brake pedal will pump up but when released the brakes  bleed down.  The brake system has been bled several times and there are no leaks. So,  why won't the brakes hold pressure?  My head is bald thinking about this one.  Thanks,  Dan