Not sure if this is your issue, but years ago when a generator was repaired, even with new brushes (and attempted seating) the compounding was so far off that the unit would stall in one direction or another. One trick we used was getting a piece of clean pine wood about the size and length of a paint brush handle and run the clean flat end over the running commutator bars to glaze them over a bit. It worked, but the series field then had to be adjusted for good up/dn leveling, and rechecked as the brushes truly seated in.
