RE: Power Brakes
what do you mean by "gas it hard"?
If you're doing a burnout and you pump the brakes a couple of times to creep forward and leave some nice stripes, it'll take a few seconds for brakes to come back.
If you're just running down the dragstrip WOT you should have brakes immediately, at least for the first pump or two.
Could be a booster or the check valve. Probably a vacuum leak somewhere, possibly internal to the booster.