[buzzojoe]

I would guess that the hole in question is a vent hole or drain hole where any fluid that might get past the master cylinder pistons would drain out so the pistons wouldn't lock up.
It's NOT supposed to have vacuum sucking into it whatever it is!!!!! You have a problem with your brake booster that is allowing vacuum to get sucked through the hole! I would get it fixed correctly IMMEDIATELY! The fix that was done by plugging the hole with a screw is NOT a correct fix and is VERY dangerous in my opinion because it might cause your master cylinder (brakes) to not work entirely!