When I was a trainee erk at RAF Halton way back in the last century, we were taught that Dry Powder was the only acceptable extinguishent for a brake fire and that CO2 would cause a massive explosion due to the rapid cooling effect.
Later in life I saw an RAF Phantom's brake fire being put out with the afore mentioned unacceptable CO2 by the guy doing so blasting the stuff onto the concrete and letting it drifted downwind over the fire.
It was unusual for the brakes to catch fire, usually just smoking after a wheel change due to excessive grease being applied.
TTFN
Howie