Brake pedal goes to floor with engine running

Plug mc ports and apply brakes. If pedal goes anywhere, its your mc. Plug rear port of distribution block, same test to isolate circuit. Gravity bleed works well, just dont let reservoir run dry. We had a 64 300, brakes sucked. Turned out to be rear adjusters, no where close. Tighten, pump and tighten again until you feel it dragging. This was a 4 drum car.