FYI:
Even if there is no dirt in there, a metal-to-metal seal is not going to be dead tight. If you don't turn off the fuel at the petcock, the constant downward pressure of gravity will cause gas to seep past the needle valve and fill up the bowl if the engine is not running. You can get a bit of overflow seepage even under the best case if the petcock is ON. The moral is to always turn off the petcock when you park it or you may see a puddle of gas on the ground.
As for the brass seat: brass tarnishes (oxidizes) which is why they start out shiny and after a few months will look dull and rough. It is a good idea to polish the seat regularly to keep the mating surface smooth (the area that mates with the needle).
It's also a good idea to put the needle in a hand drill and polish the tip up real smooth. That gives you the best seal you can get between those parts, but they can still seep gas if the petcock is left on.
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