the caps are vented. the vent in the cap keeps it from building any pressure. the vent in the cap keeps the tank from going into a vacuum from the fuel pump sucking on it. even with the cap off you could still have it leaking where you say it is. head pressure in the tank is causing it to leak at one of the fittings. what i like to use on my fitting that i use thread sealant on is this. http://www.ferguson.com/FergusonSearch/RecordDetails.action?R=PROD_136577&search=true

or you can use something of the same sort.