Your brushes may look ok . . .however, the shorter they get, the less spring pressure on the armature. I like to keep my brushes good and new. I replace them before they start jumping around, maybe earlier than necessary, but I don't have a lot of generator problems either. When your brush is worn, all that material worn away from the brush has turned to dust (and it's all over inside your generator to gum things up).
Real exciting stuff on this blog ! PS. Take a photo before you start unhooking all your wires . . .