I found that the gasket between the valve and the shaft that turns it goes bad and leaks coolant into the mechanical (top) side then flows out of the drain holes or out of a common crack. DON"T open these things unless you have something to reseal that gasket. Ask me how I know, its a PITA.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section