Unless I misunderstand the system, the diagnosis is missing something. If you have a dual master cylinder, even if a brake line fails you should have brakes on two wheels since the brake line systems on front and back should be independent of each other. If you have no brakes at all, there is a problem with both front and back wheels somwhere. Perhaps the comment about the master cylinder is correct and that is the additional problem.
Let me venture a guess. If the car had been pulling to one side all along, the brake line had been leaking all along with only one wheel having brakes in that part of the system causing the pulling. When the pedal went to the floor, something in the master cylinder failed causing the other set of wheels to lose their brakes. You can rebuild the master cylinder.