We try very hard to keep our bilge dry and are usually successful. Yesterday I was cleaning the interior of the boat (yes spring comes early in SoCal) , stem to stern. When I got to the galley I noticed the water tank valves. Remembering that we had been operating on the rear tank, thought it might be time to drain down the forward tank and opened that valve. Later in the afternoon, I went to a friend's boat for glass of wine, came back and there was 3 inches of water in the bilge, in fact the pump came on. Grabbed a flashlight and quickly determined there was a steady stream of water coming from the forward water tank.
We had known that the cap on the top of the forward tank (1987 model) was prone to leaking. We usually underfill a little bit accordingly. What happened yesterday was that with both valves opened, the rear tank was filling the forward tank- all the way to the top and a little more. This was discussed in a previous thread..
http://c34.org/bbs/index.php/topic,2909.0.html (http://c34.org/bbs/index.php/topic,2909.0.html)
So if you have a mysterious leak and both valves are opened, it is worth a look. It is also a good reason to operate on one tank at a time. Now to fix that cap...
2nd ; Yep, you're right!!
So you tested that the aft tank WILL drain (by gravity) into the fwd (starboard) tank and your fresh water system functions as designed.
The way that you seal the 4" clean out on the starboard tank is to use some non hardening thread sealant and retighten!!
A thought