[hharney]

That is an interesting issue. As for the test on the tank drain, if you are referring to the service port drains you need to check them again. Here is the method I use:

First, use Avgas, there might be a leak and you don't need to introduce more water into the tank.

While introducing the Avgas to the service port drain, have a second person plug the drain below the wing to be sure there is no cracks or leaks that would allow intrusion of liquid into the tank.

Your model has a different fuel system than mine. Mine has a sump in the boom and that is where the fuel sump is. Cessna eliminated this sump in the boom and I will need to take a closer look at the diagram to remind myself of the system.