Hi Larry,
You might try to find a product called "Mr. Metal". It works well on many types of metal, including brass. I've used it with a Dremel as well as hand polishing. One of my local hardware stores carries it.
FYI: the switch and the one-way valve aren't brass... the original finish on them is galvanized.
There are other types of brass polish that would likely work, too.
After I get something like that cleaned up and looking nice, I wash it with disc brake cleaner and/or acetone or lacquer thinner to remove any and all traces of the polish or oils, then coat it with clear engine paint. That holds up well against oil and fuel (but disc brake cleaner will remove it) and protects the part from new tarnish or corrosion.
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