Original caps didn't have a valve. New ones do and need to be disabled. For some reason the aftermarket put valves in their caps unnecessarily which causes the development of a vacuum which restricts flow to the fuel pump, and/or causes high pressure in the tank as the day grows warmer causing fuel to puke out once the valve release.
It's an easy fix. The bottom of the new caps are usually plastic. The valve is in the center. Drill a 1/8" hole just to the outside edge of the valve. That will allow the tank to vent just like the originals did. You still want to avoid filling the tank all the way to full unless you are on your way on trip where you will use a gallon or two of gas right away.
