Having a clean out in my workshop and came across a little steam engine I built many, many years ago, it is a 1/2" Bore single acting slide valve.
I made this engine as a first project when I had a small Perris lathe, the drawing for the engine was in a book titled Scale Model Ships Their Engines and Constrution no castings were needed and it was a ideal first time project. I had the engine in a model boat at the time and it worked a treat.
A pal of mine who restores brass microscopes, chemical balances etc, swears by a dunk in Harpic to shift the tarnish, before the Brasso and elbow grease.
It's probably the hydrochloric acid in the Harpic which does the trick, so brick cleaning acid would probably do equally well.
Thanks for all the cleaning suggestions for my mucky engine, I will get around to cleaning it sometime, here is a picture of one of my cleaner engine's