The phrase originates from the fact that military personnel, especially brass bands, are expected to keep their brass instruments gleaming. This translates into a wider expectation of cleanliness and presentation for all military personnel.
So, "to police his brass" implies that someone is:
* Keeping themselves clean and tidy.
* Maintaining a high standard of personal hygiene.
* Paying attention to details like their uniform, equipment, and appearance.
The phrase is often used in a humorous or satirical way to comment on someone who is overly concerned with their appearance, or to highlight the importance of hygiene in a military context.