Fuel Sender Leak

If you source an o-ring from local, make suee its viton or at least nbr. EPDM is for brake systems, but no good where fuel is involved.

It can be a pita getting it passed those notches. They are like a lock washer and prevent it coming loose. Not that mine would rattle loose. It was hard all the way. I ended up making a tool out of tube similar to exhaust pipe and used it like a tube socket with a screwdriver through it for a handle.

Like Bart already mentioned, if its leaking through the terminals, time for a new one.