You have been lucky then possibly ?
I have replaced brushes in a few older models, if the commutator looked good after a good clean that is.
I have replaced brushes in a few and they failed shortly after though, however with the labour involved on some models - i think that is the reason Stuart suggests to replace the motor, as well as a more gauranteed lengthy repair for the customer.
i will admit that some customers in the past i didn't want to see again, so replaced the motor and get rid of them for a lengthy time.