I have found after owning several MBX5 versions that the center diff top plate brake bushings SUCK. After several long mains with heavy braking the bushings are blown and the lever arms become sloppy...i suggest the optional hopup top plates with bearings...King Headz comes to mind.
Also, the brake cams tend to wear rather quickly (IMO) causing the brake engagement point to change....also causing you to either flip it if you havent already and buy a new one if youve already smoothed out both corners of the cam lobe. This makes setting the brakes a pain.
In the end, i would also swap out the standard steel rotors for some G10 units and scrap the "pads" on the shoes. This guy makes some killer G10's for the Mugen.
Look up "xrayeddy" on the forum and shoot him a pm for some.