My first guess would be the master cylinder is the culprit.
There could be air in the lines if they were not bled properly, but I would think that if continued pumping of the brakes does not build pressure then this would indicate the master cylinder is unable to build pressure.
If it were a problem with the booster or a vacuum leak to the booster, the pedal would not got to the floor, you would just have to push the pedal really hard to stop the car, but pedal travel would not increase.
Assuming no brake fluid leaks, no air in the line, proper level of fluid in the reservoir, etc, I would be inclined to believe the master cylinder is at fault.
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record