Brass colored dust? Are you sure someone didn't add some of that copper stop leak or any other stop leak for radiators? What you should do is drain out all the coolant from block and flush it all out with a hose. Once only water is coming out clear, drain the system completely and only run distilled water. Now you should be able to identify if in fact your head gasket is leaking if your water begins to accumulate oil. It's also easier to identify the presence of oil in water vs coolant.

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1990 190E 3.0L