heres a photo of the gas cap of my 69'. i don't think its original but its old for sure, atleast 25 years old. it sais "Vacuum Pressure" on it so i'm assuming it works the two ways, or it did when it was new. i had a boat once that the tank was not vented and after a while of driving, the engine would stop running because it was starved for gas, as soon as i opened the gas cap, it would do what your saying "whoosh" and the vacuum inside the tank would relieve. i guess drawing all that fuel out of the tank when its' not vented, creates that huge vacuum inside the tank. as for pressure in the tank, like the others are saying, if it sits and gets hot in the sun for example, the pressure from the vapor inside the tank builds and has nowhere to go, then it finds the weak spot (sender gasket, etc...).. i would say you need it vented for sure for both vacuum and pressure, but thats just my 2cents. i like the idea of putting a small hole in the cap, then you have the vacuum/pressure vent just to be sure.