[folivier]

One of the most common areas for a leak are where the filler neck is soldered to the tank. A few of us have had success by wire brushing that area clean and using JB weld epoxy to repair it. Or if you have time pull out the radiator and have a new neck soldered on at a radiator shop.
Do you have an overflow tank on the genset radiator? Easy to add one, most auto parts places carry them. The tank makes it easier to keep track of the level.