hissing when hitting the brake pedal

[jklonowski]

my truck is making a hissing noise when i hit the brake pedal the idle raises which clearly is a vaccum leak. well i was told it was the vaccum booster, well i replaced it today and it didn't take care of the hissing when i hit the brakes. i don't know what it is so a buddy and i are going to tear into the dash. it sounds like it's coming from behind the ac controls above the radio but i'm not sure. i can't hear any kind of hissing under the hood but i also have headers to cherry bombs to 3 1/2" tips in front of the rear tires so it's kinda hard to hear anything. any ideas where the leak could be coming from would be greatly appreciated.