Im having a issue with a leaking sending unit in my 240 Sea Ray Sedan bridge. The 70 gallon tank inside the boat under the floor has a leaking gasket. Ive replaced the gasket with a actual sending unit gasket (rubber or viton, not sure but it is black) and sealed around the perimeter of the gasket and screw heads with fuel rated sealant availble from a major OEM designed for gasoline use. Now three years later its leaking again. The fuel level builds up in the filler hose above the tank so the gasket sees a little bit of pressure from the water colume of fuel above it. the sender screw were torqued to 10 inch lbs also. Im at my wits end on this and yes I know its very dangerous, hence why im fixing it . Anybody have a fool proof way of fixing a leaking sending unit.

Sure it’s not thru the screws ? The gasket is so thick, seems near impossible to leak around flange if correct screws and decent tightness.

Don’t fill it up into the filler neck

On my previous boat, I had 2 shops fix my fuel sender, which was leaking. Turns out the issue all along... the screws were too long. Finally found a good mechanic and he put some washers on each of the 5 screws. never leaked again. The screws would bottom out, and be 'tight', but not actually pulling the sender into the tank.

When I pulled my sending unit to check my alum tank, I noticed the fine thread machine style screws don't really have a seal system on the threads. Doesn't seem to be an issue for me, but the screws are so small and fine, that any sealant would likely add too much removal tq and break off the heads.

Maybe a little bit of fuel safe rtv style under the machine screw hex heads? Or a sealing style gasket washer?

What I'm getting from your issue...is the tank is getting overfilled, and you have some head pressure in the tank, causing the sending unit to weep/leak?

Is the surface of the tank flange flat? 3 years later the adhesive lets go and passes fuel? How impervious is the sealant to fuel...and for how long?

In addition to head pressure mentioned here; if you have vapor pressure building up and not being released by the breather line (happened to me when full/overfull) the path of least resistance is to push gas through the gasket and/or bolt holes as you are experiencing. Check your vent line is clear especially at full tank.

Coincidentally, I was dealing with a similar issue this last week. I'd replaced the original float-style fuel sender with a reed-style one, followed the "do not overtorque" instructions to the letter and...it would weep when the tank was full. As a stop-gap measure the last time I used my boat, I'd put a bead of PERMATEX #2 around it and it worked...for a while. So a few days ago I tackled the problem again and simply torqued the screws down more, probably another full turn or two, and...fixed it.