Rick,
thanks a bunch for your suggestion- I agree about the ingeniousness of it! Only issue I have is that it necessitates filling the tank. Since I have not yet closed on it, I am not 100% keen on the ideal filling the tanks for essentially the seller.
is there another way to test it that does not require filling the tank with fuel? On top of that, I would be concerned that if it is a fuel leak that filling the tank could result in a lot of spilt diesel in the bilge which would be a real mess to clean.
That is where I thought of lightly pressurizing the tank with a gauge on it since I can't see all around it and check via soapy water. If tank is drained, all connections sealed off, pressure gauge installed at inlet where air is pumped, and then slowly pump it by hand to like 5 lbs pressure and watch how it acts an afternoon or a day, would it not tell you if it had a leak via drop in pressure?