Francois
The Carbon Brushes need to be re-seated to the commutator, which is a simple and easy thing to do. But, you'll need a piece of carbon brush seating stone. A local vac shop should be able to sell you a small piece which is all you'll need. With the machine running, hold the seating stone against the commutator close to the leading edge of one of the brushes. Press against the commutator for a few seconds, all the sparking should now be a very minimal of just a small line of blue sparks right at the leading edge, with no 'fire' ringing the commutator. Because I own a vacuum shop, we use these seating stones all the time when working on machines where we have to take a motor apart, like a Rainbow or an Electrolux or Kirby.