I had a torn diaphragm in mine which made the pedal go soft to the floor with minimal braking. It required a real hard push then. Had the diaphragm repaired and that essentially solved the problem ( I say "essentially" because I think I still need to bleed the system a little more). The torn diaphragm was caused by fuel backflow through the vacuum system. The anti-backflow valve (real name escapes me at the moment) was replaced also. As far as diagnosis goes, I knew something like that would be the problem because there was an audible hiss when the pedal was depressed (vacuum leak) and the engine would run rough. That's one possibility to investigate. Cost me $125 to rebuild the booster.