You would not take the back gear off. You would hold the shaft between the commutator and the motor frame with the 9mm wrench.
Start at 6:30 in this video. However still use the thin tool in the hole of the shaft. Pliers can damage the shaft.
Or 14:15 of this video:
I watched both of these before tearing mine down so I knew what to expect. Pretty easy machines to work with so far.
Let me know if you still have any problems.