In a previous thread, I posted the procedure I use to seat carbon brushes with a seating stone.  It's actually straightforward and easy to do.  However, with older Rainbows, there is no access to the commutator so that the brushes cannot be seated in with a stone.  Someone suggested running two motors in sequence.  That will slow the motor 50% of it's normal speed (which I did not know).  I think the suggestion was to run them in sequence for 15 minutes, which I did.  The motor on the right is the one with brand new brushes.  I then took the motor apart again to inspect the brushes, and it appeared that 15 minutes wasn't enough.  So I reassembled the motor and ran it for another 15 minutes.  When I first began running the motors, the power reduction was noticeable.  But after about 10 seconds, I noticed the motor speed in one of the motors increased slightly.  I guess the brushes already were making better contact with the commutator.  Well, I didn't take it apart a second time for an inspection, because it is a big pain in the rear to take apart and then reassemble one of these motors!!!