DON'T use Brasso! It's too abrasive, and gunks up any crevice it gets in.
Flitz or Simichrome on a soft cloth will shine it. There is also a product called Nevr-Dull that will work. Lead Away cloths will work, ut be very gentle! Once it gets shiny, stop polishing. A Dfariswheel says, you can cut right through the plating.
With any of these, spray the gun out thoroughly with brake parts cleaner & dry thoroughly, then WD 40 & dry thoroughly, then lubricate.