The heater matrix is blocked so am replacing it tomorrow (joy of joys) but last time I did a coolant flush i just emptied and refilled, within a few days it was brown again but it's not losing any coolant, and it's not oily at all.
Thinking it's general rust and crap settled in te system? The car was stored for a year before I bought it so could have built up then.
No idea how bad the matrix is until tomorrow but would a backflush/rad flush cure it?
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Bye bye Sundance Kid