Power Disc Brakes not working on hard stop?

Dartman,
Read post #14.
Given the accurate & detailed description of the problem you outlined, I am simply amazed at some of the illogical suggestions [ such as p'rod adjustment when you clearly stated that the brakes start applying with 1" of pedal movement ].
Other than a defective booster, there is only one other thing I can think of that could cause this problem: corrosion in the m/c. On light brake application, the m/c seals against a smooth bore & max pressure is developed. On hard braking, seals move a bit further into a corroded area of m/c bore...& fluid leaks past. I think this is unlikely, but it is possible.