I was just wondering this, but I don't think its going to be easy to answer as you would need to define what a brass band is/was. I have played in two very old bands. Penicuik Silver (1835) and Alloa (1842) but both of these started as military style bands with woodwind in them. Many of these bands switched to brass only by the late 1840's. Even then the instrumentation was not what we have now. At Alloa we had old photos on the bandroom wall showing the band in the 19th century with lots of flugels and not many cornets. So, I suppose there are really two questions: 1. Which current brass band was founded the longest time ago (regardless of original instrumentation)? 2. Which of the currently existing brass bands switched to brass only first and when was that? Maybe someone has Roy Newsome's books. The answers might be in there.