hard pedal on brakes

I have a 96 4x4 and the brakes work good if I step on the pedal normally. If I do a "panic" stop or hit the pedal quickly, it's hard as a rock and they
don't work right away. Sorta feels like the engine is off and there is no vacuum asssist but, no matter how hard I hit the pedal the brakes won't come on for about a 1/2 second. Have vacuum at the booster and holds vacuum (how much?). Replaced the brakes and fluid this weekend but, only because it needed it. The problem was before that. I was thinking the booster, because the engine changes when I press the pedal and the pedal will drop if I speed up the engine. After reading Doc's trouble I started getting worried, except that once the brakes come on I can stand on it and it stops like it should. Thanks for any advice. David

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