If you fill them with water and soak them for any real amount of time, you'll strip whatever flavor in them out. Try soaking the outside several times. Then put a few gallons of hot water in and bung up tight with a rubber/silicon bung. Wait a few minutes for the water to cool then remove the bung. Of you hear a vacuum, then the barrel won't leak. If there isn't a vacuum, then try to locate the leak etc.
Filling and soaking is a last resort.
Don't be surprised if you get quite a bit of flavor from the "neutral" barrel. Beer, even the most aggressive ones, is real delicate.
But to respirit a barrel, I suggest 1/2-3/4 of a fifth .