little OT-Angle Grinder question

Hi there... I am a pipeline welder of 25 years and I had a "stack" of non working Makita angle grinders (mod 9005b) under a bench in my shop, that had let me down over the years. Last night I decided to take these things apart (with a bottle of J&B) and try and salvage a few of them by swapping parts etc. I managed to make three good ones by swaping parts. I have three other grinders that would be good ones if the brushes would quit arcing against the armature. When I took the brushes out, they were burnt on one side so I went into the city and bought new brushes. I installed them and get the same result. I took the armature out and put some emery cloth on the bearing surfaces where the armature meets the brushes, but get the same blue sparks comming out of the grinders. Is it the windings that are shot? Am I beat here? is there anything I can do? or are they off to "Grinder heaven"? Thanks for any words of wisdom on this subject....Regards...Jim
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