When you press on the brake pedal, does it go part way down, hit resistance, and then settle down to the floor? Or when you press the pedal does it sink straight to the floor?

A couple possibilities:

You may have a poorly rebuilt master cylinder. There are plenty of sketchy ones being sold. I bought one a few years ago from either Advance Auto or O’reillys and it was missing some of its internal pieces.

The rod length may be wrong between the booster and MC. Too short and the pedal has to travel too far down before it starts to push fluid. Too long and the piston doesn’t fully return home to allow fluid to drop from the reservoir down into the cylinder. Did you compare the old MC vs the new MC to see if the pocket on the rear of the piston is the same depth (measured from the MC-to-booster mating surface)?