I recently replaced the Fuel Tank and the Sender Unit on my 85 Vanagon; I purchased these from Van Cafe and had my VW mechanic install.  Initially had a BIG leak when filling up the tank.  Mechanic dropped tank and spent time bending and adjusting the tabs that hold the Sender Unit to get a better fit with the o-ring, and used a straightedge to try for as flat a surface as possible.  The leaking stopped (he pressure-tested and smoke tested)...but has now re-appeared when filling the gas tank to full.
It appears that MANY aftermarket tanks and Senders have this problem.
I'm thinking of applying some epoxy putty (gasoline resistant) when installing the Sender to fill the potential gaps of the mating surfaces.  Maybe some of the epoxy between the mating surfaces, and then build up a fillet of epoxy around the perimeter of the Sender Unit?
Any advice on this issue?  Van is currently over at mechanics....we are scratching our heads for a permanent solution.

Rich
San Diego