I had a similar problem. I found mine by pressurizing the tank with compressed air through the expansion vent tube then listening for escaping air. Be careful not to put too much pressure in the tank or the expansion trap will "pop". My problem was a small hole in the tank that came from a sheet metal screw that held down a cover. It occured while re-installing the tank after finding another leak. Good Luck.

Jim Villers
190SL, 230SL