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You can soak your stained brass in a solution of white vinegar and salt it will remove most if not all of the dark staining. Mix one quart of vinegar with a tablespoon of salt soak unprimed brass for 15-20 minutes then rinse well with hot running water and dry in a warm (200-225 degree) oven. This won't shine the brass but it will sure clean it.

Avoid polishes like Brasso as they contain Ammonia and will weaken the brass.