Is it possible that a new tank/with new sending unit may be leaking through the sending unit mounting "O" ring? I've got a little gas coming from the middle of the tank dripping onto the heat shield. I never liked the new sending unit's lock ring fit, it seemed just a tad small and wanted to walk out of the locking tabs as I tapped the locking ring around to seat it.I wondered if it was a good fit and was wondering if fuel can slosh up there and out through the O-ring.

Happens at full tank and not at half tank of gas.

 

can also just as likely be a leak from the hose connections at the top of the tank and the filler hose connection.

they all will be more likely to leak when full due to "sloshing"

 

Ended up being just that, the sending unit O-ring deformed on installation. Got a GM O -ring and lock ring. Much better fit and the O-ring from GM is steel core with alignment tabs to keep in position. Lock ring was also much heavier duty than the aftermarket piece and it tapped around just like it should have. Now NO MORE LEAKS!!