I can't seem to completely emty the bowl of seawater after first completely flushing the head. It continues to fill again to the half-way point even after pushing the valve to the "empty" position. I've tried closing the intake valve with no luck. It was suggested that this was because the head is actually below the water line, but if this is true, shouldn't closing the intake stop this problem? My concern is that I don't want the water to slop out while heeling. Any suggestions?