I am re-doing 69 mustang with 302 crate engine. I have replaced all brake lines, disk brakes, proportioning valve, master cylinder and brake booster... When car is off I have a firm pedal but as soon as I start car pedal goes soft and to the floor. I have good vacuum and booster is holding pressure. I have bench bled MC and bled lines 4 times by vacuum an by gravity. At a loss!! Suggestions??