Not silly and probably right on the money. To the OP. READ the shop manual sections on fuel (tanks) and emissions. The vent system is explained. You may have something modified, leaving what was the vent system plugged, or the cap may be corroded up. These caps ARE NOT intended for "normal venting," they are instead what is known as a "pressure / vaccum" cap, which means that spring release valves vent if the vacuum or positive pressure in the tank gets so highYou can download a SEVENTY THREE manual here for free, there MIGHT BE some differences in the 74 system.
http://www.forabodiesonly.com/mopar/showthread.php?p=1970088617DOES IT APPEAR that the factory engine bay "carbon canister" is all properly hooked up?Look for a 1/4 steel line coming from the tank area up front
