Iron fillings in valve

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Iron fillings clogged valves up to the point that the pump would cavitate making it damn near impossible to Hydrovac anything. Originally we thought it was air getting into the system from an unknown source but after getting the pump fixed it continued to cavitate and upon further investigation ( dismantling the pumping ) this is what we found.’this is caused by
several things, one being not letting the system cool down after running the furnace during the winter.