1 carbon brush wearing away on one long side forming a ridge, as if not touching commutator properly and burning causing brush to overheat and shed contact wire and spring the brush on other side is fine with even wear across contact face of brush could this be caused by brush guide wear which would mean a new casing ive had a good look at the brushes and it would appear that the faulty one is now only 14mm /11mm long on the now angled face which made contact. the good brush is still 18mm long