Need help with the brakes on my 72 mark 4 lincoln

My brake pedal goes all the way to floor. it only seems to do it when the car is in gear. Its not losing any fluid anywhere. Ive tried to get the pedal to do it with the car in park and it wont do it. When the car is in gear, and i gradually apply the brakes, the pedal goes all the way to the floor, and it wont stop unless I pump the pedal. If i jam on the pedal (while in drive and moving) everything works normal, the brakes are right there where I need them. Maybe ive got air in the system somewhere but I haven't done anything to the brakes on this car. Ive owned it for 12 years.

Sounds like the master cyclinder is leaking by when pedal is applied easy.Put a new master cyclinder on it and see what it does.

Whats the best way to bleed my new master cylinder? Ive seen one procedure where the plastic brake line hole caps are used, and that's it. I have also saw one where they use the rubber clear hoses, and run them back up into the reservoir, from the brake line holes. Which one is the most foolproof way to get it done? The old brake fluid was filthy, there was a black film coating the bottom of my old cylinder. when i took the old one off and dumped it, the old fluid was almost the color of used engine oil. When i reinstall this will all four wheels need to be bled also? I assume they will. Just asking to be sure. Thanks.