It pays in the long run to get the correct factory brushes - too soft and you carbon up the motor - too hard and you wear out the commutator. I've heard of people using paper to establish "hardness" but I wouldn't trust it with my tools, and especially not with my employer's motors!
Don't know if the following links might help.
http://www.ebcis.co.za/carboquip/Brushes.htm http://www.toolmarts.com/repair.html
