Grigg makes a good point.
Many of the off the shelf "vented" caps comply with the EPA standards to reduce gasoline vapor. They have a one way valve that lets air in as the gas level goes down but don't let anything out. I've seen huge pressure buildups in some tanks.
I remove the spring from the valve if I can't find an old style cap.
And yes, don't fill the tank above the bottom of the filler neck unless you are going to burn off a couple gallons before stopping.
