do i have a bad brake booster?

[1FaastC10]

i've noticed a few things with my 91 K1500 lately. the brakes seem to be grabbing evenly, and it doesnt pull either direction when they are applied. i am however experiencing a low pedal. today, i applied the brakes (while it was raining) and the pedal pulsated pretty violently as it went towards the floor. it stopped just fine, but made a hideous noise along with the pulsation. i've also noticed that if i step on the brake pedal before i start the engine i hear what appears to be some sort of a vac leak. if i dont step on the pedal before i start the engine its fine. anybody got any ideas what my problem may be? i'm all sorts of confuzzled here.