is it that top fixture that's 'leaking'? needs a new seal easy enuff to make your own out of thin cork gasket material - but if you're getting actual fluid out there you're way over-filling the tank, which will also result in fluid running out the fill tube [past the vented cap with the brittle gasket] as the cool gas from an underground tank heats up in the truck and expands, common complaint here actually: "why does the gas puke out the fill tube and ruin my paint?"
also check the rubber hoses from the tank outlet to the fill tube and vent, they can get brittle and crack with age, or the clamps can be loose - that tubing is a special 'gas line' tube if you need to replace it, don't just use whatever fits
Bill
