A vented cap should never allow pressure or suction to build. In addition, many cars had either a vent port and tube in the filler neck or under the trunk drop that would allow internal and external pressure to equalize. My '69 has this, as well as its original "VENTED" cap. In later years, these tank vents were routed to charcoal evap canisters, but mine is just an open tube.

If you replaced a tank or filler tube equipped with a vent with one without a vent, you will have issues.