I had the exact same X-star for a while (same color and everything) and had a similar intermittent issue with the leak into the ski locker. I don't think mine had anything between the thru-hull ballast intake and the locker except for a bilge pump. It would pump and fill alright, but then it seemed like it would continue to siphon even after turned off. A valve in the line seems like a good, safe way to go but accessibility and hassle is the issue. Replacing the bilge pump with a reversible ballast pump may do the trick. Also just putting a section of the line between the pump and locker at a higher elevation may help if it actually is the same issue that I had of course this would probably require replacing the line with a longer one. I don't think that I had any leaks anywhere.