[RichardJ]

How well is the booster working?

The vacuum boosters are designed so that in case of engine failure, there should still be some brake assist for three applications of the brake pedal.
With the engine running and pressure on the brake pedal. Turn the engine off. The pedal should remain in that position, but repeated application will produce a higher, harder pedal.
Hold pressure on the pedal and start the engine. The pedal should drop. The booster is multiplying your foot pressure and pushing the pedal down.

The MC used with a booster usually has a larger bore than with manual brakes. It's possible the MC bore is way too large or the booster isn't working as well as it should.

>>.I about threw myself through the windshield.<<
Elaborate on this. Does it happen at all speeds and with all applications?
If this is a low speed problem such as in a parking lot or while parking, the problem is with the Metering Valve inside the Combination valve.

Could you provide another description of, "pedal seems stiff".