Re: 1989 Hunter 40 - A million questions.
My vote is for the tank vent. I think Jeff spoke correctly about warm, expanded fuel being sent back to the tank, and over-flowing out of the vent.
The reason I say this, is because the problem did not surface until you topped off the tank. Otherwise, my money would be on the fuel return line.
Run the engine under load, and observe the tank vent (if possible) to see if any fuel is burping out of it. I'll bet that after you consume a little more fuel, the "leak" will stop.
S/V Old Shoes
1973 Pearson 30 #255