(starter) motor failure

[madderscience]

OK, this is NOT my MR2 (happily) but in fact the starting motor in my (very non-green) 1962 land rover.
However there are way more motor experts here than in the rover crowd so I will post my question here.

The starter motor in my truck (which is basically a scaled down version of any of the DC motors you see in conversions, 4-brush cylindrical commutator, series wound, etc.)   failed recently and upon taking it apart it turned out the commutator and brushes had failed badly.  This occured fairly suddenly (within the course of a few starts).   The commutator seems to have started flaking off bits of copper which shorted out the bars and fouled the brushes (which are still intact though), damaging it to the point where the whole thing shorted out and quit working.  There wasn't any sign of carbon scoring or excessive heat but perhaps this was the result of arcing or what have you?   The brushes weren't even fully seated yet though this starter was rebuilt 5 years ago.

Ideas?  Don't want it to happen again, or to have anything similar happen to my EV.

-Brian