You could probably test the sender with an ohm meter or continuity tester.
Remove the sender from the tank and dry it.  Put the red prong on one tab and
the black prong of the tester on the other.  Now check for continutity.  It
should have none.  Now dip the two prongs into a glass of water and there
should be continuity.  I am pretty positive that this is how this works.
My experience has been that usually this seeping is due to very tiny cracks in
the tank where the sender screws in.  You should be able to see them with a
flashlight and maybe a magnifying glass.
Let me know what you find.
Ken Wilford
Van-Again
John 3:16
PS somebody correct me if the sender is supposed to work some other way as I
am just thinking that this is how it should work but I am not positive.