IF you determine it is in fact the cap (don't drill it yet)
make a decision if you want to add a charcoal vent in the line so that you don't get the gas smell around the window and in the garage.
It will vent thru the cannister in that setup and the cap will need to be like it is not venting out or in. The suction back thru the cannister pulls the fumes and clears cannister as well.
if you going old school stock setup with all the smells that go with that because the is what you want. then drill or change caps or adjust springs however you desire.
How would one install a charcoal vent system in 70 year old truck with the tank behind the seat? Will this charcoal vent system also be incorporated with the road draft tube and the open vents in the valve cover? There are likely much nastier fumes coming from those items than from a free-breathing gas cap.
