My friends mom was driving and hit the brakes and went to the floor first thing I thought of is Master Cylinder but I pop the hood and the fluid is full, It also looks like it'* been replaced before, I don't see any leaks from any of the lines or the MC Itself. So I then decide to replace it anyway just incase I do so and problem is the same. Anyone have any other ideas? MC is new, Lines aren't rusty and leaking at all, Brake Fluid is full.

How are the pads and shoes?

Did you bench bleed the new master cylinder before installing it?

Did you bleed the entire brake system?

I am also in the camp of checking the entire braking system. It'* not complicated, and should be easy to track down.