+1 for the master cylinder.
How did you bleed the master with the pedal? What was your process?
Is there ANY resistance in the pedal at all lower down, or is it just stuck to the floor?
I'm sure you're already aware of this, but you can bleed the master on the car by press and holding the brake pedal to the floor, crack open the lines to the master cylinder to release any air, closing the lines and then release the pedal - then repeat. No pumping is required between these steps, just:
Hold pedal, open + close lines to master, release pedal > repeat.
But if you have zero pedal resistance throughout the whole travel, you're probably best to remove the master and bench bleed it.
Although frustrating, it will likely be an easy fix.