[WSSix]

The vacuum booster assists in applying the brakes. Low vacuum would make the pedal harder to push as if you had manual brakes.

This sounds like a bleeding issue. Sitting in the drive way, can you pump up the pedal? By that I mean, press the pedal steadily repeatedly. Does the pedal get harder? If so, you have air in the lines. From your description, this is the first thing I would check. Sometimes, you can have a tiny leak that wreaks havoc on your system. Or, it could simply be a system that's going to be difficult to bleed.