Try the following: Assuming that the field coil and armature are in their places in one half of the clamshell along with the ball bearing behind the commutator, lift the brush along with the brush holder out of the clamshell, fit the replacement brush into its holder, compress the brush into the holder and insert the holder into the brush space in the clamshell, with the brush touching the commutator. Do the same with the other brush. Making sure that the brush holders are not shaken in the process, assemble and engage the other half clamshell to trap the brush holder.
Hope this helps.
Chandrashekhar from India, April 2012
