Pulled my 1975 glasply 21 fter 302 mercruiser i/o out of salt water last weekend. Have been working on winterizing. Ran some salt-a-way with muffs couple of days ago. Has a closed loop fresh water system. Pulled the plug on the bottom part of the heat exchanger and tubes to/fro the H/E...did not get a drop of raw water. Should i have? Certainly want to drain raw water. Read a post that the risers have a plug...i see a large allen wrench type of plug on the side...would i find raw water there? I'm reluctant to pop that plug...have to head into town and get a much larger allen wrench...and if it isn't what it looks to be. Help!