Brian,
I have been experiencing the same issue on the unit I purchased about five years ago.
While I have no information on any improvement or redesign which could have taken place recently, I found that one can mitigate the problem by using partially depleted batteries to reduce the strength of the motor driving the brush. I suspect that the use of 1.2V rechargeable batteries might help too, since it would reduce the voltage.
Also, when using the motor, one should make sure that the brush is held vertically, with the brush on top, to reduce the likelihood of the brush flying off the pin.
Cheers,
Fabien