Christian,

It's to reseal the various connectors and hoses on top of your gas tank.
Various listvendors sell the pieces to do this work yourself. Ken at VanAgain
even has a webpage dedicated to it.

After a time, all those rubber grommets, hoses & gaskets for the fill pipe,
overflow hoses, etc., do go bad. For those in the rust belt, you may have
worse trouble in the form of a rusted gas tank. Only way to know for sure is
to drop the tank. If not rusty, just replace the various pieces and you are
back to filling your tank to the "full" mark.

BenT
 <A HREF="http://hometown.aol.com/bentbtstr8/myhomepage/index.html">Vanagon
Cafe</A>