I am leaning toward something chemical going on here. If the cooling system had 16 PSI in it when at normal operating temperature, then cooled down to ambient temperature, the pressure would drop to below atmospheric pressure ant suck coolant from the overflow tank. I think the next step would be to drain and flush the cooling system and fill it with clean coolant and distilled water. It is scary to think that some chemical in your cooling system could be eating away metal. Maybe this has something to do with your water pump going bad. Did you completely drain the cooling system (including pulling both block plugs) when you changed the water pump? What do you guys down under put in your cooling system other than water?
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