Some of tanks were vented to the charcoal canister on cars equipped with them.
When eliminating this type of system I usually buy a vented gas cap. Most sizes are available at parts houses or if it is a unique design or that is special to the car or truck a small hole can be drilled above the cap sealing gasket into the the center (tank side of the cap). Unnoticeable but functional. All the tanks one way or another have to be vented to breath. I have seen tanks with a plugged breather literally suck the bottom of the tank up before the car stalled from lack of gas.