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Hi sjc115
Have you already checked the motor is rotating freely? (Excess friction can stop motors)
When you checked out the brushes, did you check the brushes were still long enough so the springs were still pressing on the brushes? Some types of brush holder have a slot that the spring runs in, when the spring gets to the end of the slot it cannot put any pressure on the brush, even though there is some brush left. New brushes will fix that.

Have a look at the armature. (This can be seen through the ventilation holes of many motors, if your motor hasn't got vent holes in the can, the motor will have to be dismantled) Are any wires off? Have the windings on one (or more) of the poles has turned dark? Unless you are into serious motor rebuilding, fixing either needs the armature replacing. Armatures are not often sold separately in this sector of the market, so that means a new motor is needed.

If none of those suggestions are the answer, possibly there's too much debris for the contact cleaner to clear. You could dismantle the motor, try cleaning out the debris from the slots in the commutator, and cleaning out any debris from inside the endbell.
Inspect the commutator for any wires off that weren't visible before taking the motor apart. (If it was visible through the can hole, most likely its beyond repair). If a wire has come unsloldered and there is enough wire left to solder back on the commutator tag, that's worth a go. That can be a symptom of a fault in the windings, in which case resoldering it won't do much good, but there's not much lost giving it a try.
 

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Hi sj115
One more idea that might work-

How about checking the brush gear alignment.
If the brushes are far enough out of line so they can both touch one comm segment at the same time, it'll produce a short circuit which sounds like what you've got. It's plausible that when the motor was reassembled the brushes displaced enough to run for a short time till they bedded in again and the problem returned.
The brushes should be worn into a curve where they meet the comm, this should be even each side. If they are wearing off centre, - if possible realign the brush holders - if that's not practical, cutting off the corners where the brush touches the comm will provide a cure till the brushes wear down again - then it'll need doing again.
 
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