What is the context? Is it talking about someone in the military? "Brass" can be used to refer to high ranking officers (due to the shiny brass medals on their uniforms).
I haven't heard "more brass than brains" before, but, "brass" is slang for "money" in the Yorkshire dialect. I would venture to suggest that it might mean the same as "more money than sense", which is a common expression in BrE.