Because old brushes can infuse moisture and oil, the already mentioned tension issue, and the mentioned carbon deposits on commutator. A brush change normally includes proper cleaning. It definitely can effect the old pulmore motors and how well they perform. Same to a degree with old PW pick ups. Running AC motors and once pickups attract moisture notice how some engines spark constantly at the pickup with rapid wear. I have a small MPC alco in that phase right now.
Can happen to MTH pickups also, I have seen plenty of pickups that have an ok appearance other than a darker ring around contact with rail, but completely erratic at providing good middle rail pickup. Changing pickups and issue is resolved. G
