[Rust_never_sleeps]

Sounds like a pedal height problem. Too much travel before you start putting pressure in the system, and you're running out of travel before you bottom out the MC.
Is the rod/rod gap correct between booster and MC?
If so, the pedal height adjuster between the pedal lever and the booster could be way too short.

Booster failure usually provides no boost, so a hard pedal and no drop with engine vacuum, you are correct. Seems like yours is functional