What would cause the brake pedel to go to the floor after front right brake line busted but master cylinder was replaced bench bled and all wheel calipers have been bled furthest to closest with barely any pressure build up brakes where pretty good before line brust

Something wasn't bled properly.
Normally, its
Bench bleed the master, then
Right Rear
Left Rear
Right Front
Left Front.
Then you need a Tech 2 diagnostic tool to do an ABS bleed procedure. This works out the trapped air inside the ABS module.

And that'* also assuming you didn't get a bad Master Cylinder out of the box (that'* happened to me actually).

Why was the Master Cylinder replaced?

But now that I'm thinking about it more, even if everything was bled properly, he should still have some brake pedal back pressure. It shouldn't go down to the floor like that even if the ABS module has bubbles.

Rebleed everything. Again. And. Again.