Your tank has to breathe. In order to breathe, your tank must have a vent somewhere that will relieve both vacuum and pressure. Gasoline expands in volume as it warms up and shrinks in volume as it cools down. The fuel level of your tank changes throughout the day, even if you are not driving it.
You cannot put fuel into your tank, unless you can get the air out...and you cannot withdraw fuel from your tank unless you can let air in.
Up through the 1960's, most vehicles used vented gas caps. This is simply a gas cap with a hole in it. Unfortunately, this hole would allow the fuel to splash out when accelerating or turning a corner.
If you are running a vent line it is important that the line is ran higher than the highest point on the tank including the fuel filler neck. Also, the vent line cannot have a dip in it where fuel or condensation can get trapped in the line. If fuel becomes trapped in the line your tank will then build pressure or vacuum until there is enough pressure to purge the vent which will cause gas and/or odor to come from the vent line. If enough pressure builds up damage could be caused to your tank.
