Fuel sending unit gasket
To your question, filler up.
You should get more than 3 gallons into the tank before the fuel gets to the sender.
I replaced my sender with a nos unit.
1st fill-up leaked.
I drained down the tank and took the lock ring off, I had used the new gasket but things were loose with it. I ended up putting the old, but in good condition, and the new gasket on. That did the trick no more leak in 4 years