I doubt it is the vacuum pump, but could be. The piston inside the master cylinder has two
"O" rings which prevents brake fluid from passing backward behind the piston when
pressure is applied. Damaged rings could cause this problem. The rings are often
damaged with brake service is performed and the person doing the job has someone
pump the brakes while they are bled. The piston inside the master cylinder never travels
it's full stroke when in use, even emergency use. Pressure prevents this. Therefore,
corrosion can form at the further end. If the rings come into contact with the corrosion,
they can be damaged causing a backflow of fluid, causing the pedal to go to the floor.