It's pretty hard to tell without seeing it. You say the brushes looked O.K. I haven't done a Genny for a long, long time but the Brushes should be about 3/4" from memory.
Even though they might look O.K. but are down quite a lot it would be best to change them. For peace of mind get a Buddy of yours who's Machenically / Electrically inclined to have a look. When Dave gets on he'll be able to go one further.
Chris....From the Land of OZ.