Originally there was a round cover that covered the sending unit. It was shaped with a rib to mate up with the rib on top of the tank and it would cover the whole indentation that the sending unit sets in. Most always this cover gets lost. The sending unit gasket often gets dry and cracked and will leak. Another place to look at is the small pipe coming out of the tank on the drivers side if yours has one, its common for them to crack where it attaches to the tank. While your at it look at the hoses going to the filler neck. Hope this helps. Larry