John, Back a season or two ago I had a very small coolant leak that turned out to be the overflow bottle itself. The wire bail that holds it to the bulkhead had been overtightened (possible because it is tightened into the soft insulation) and it developed a leak in the area of the wire bail. When I replaced it I added nylon spacers between the bail and bulkhead so that you can't over-tighten. I also added a strain relief clamp to hold the hose to the bulkhead just below the tank. There is a small (very small) vent hole in the top neck of the tank just under the cap. If the tank was too full and was sloshed around in rough seas you could loose fluid (thou a very small amount) that way as well. Others have had leaks that weren't so easy and took much persistance to find. I believe most of these were found at the heat exchanger or the hoses going to it. Good Luck and let us know what you find. Gary 2003 C310 #230
