I would think the Master Cylinder is the reason the pedal goes to the floor, not the booster.
Nick
I agree, provided properly bled. The firm pedal engine off usually rules this out. A sinking pedal is a sign of losing hydraulic pressure. Typically caused by a leak. Externally which could be seen or internally in master cylinder as fluid bypassed cup seals. You could remove lines from master cylinder and install plugs to confirm this. Bleed by having assistant push pedal Down as plug are tightened.
