I am with him ,non vented gas cap on older fuel systems does not work well,it allows a vac to form ..............if the gas cap has been replaced it`s probably non vented,the trucks with the gas tank in the cab have a hard time with those non vented caps too,they pressurize and leak vapor out the seals into the cab.

You could try and drill a very small hole in the cap to test it out ,at this point a new cap is cheap.....just drill it off of the tank ,so no shavings go into the tank or worse yet ....booooom.