zoombie

why does my P2k2 pro sparks on one brush but nothign on the other ?

Steevo

I would check that your brush is not hanging up slightly. It could be that the brush hood is dirty and the brush is hanging up. Also check the braided shunt to see if it is caught. Sparks usually occur if the brush is "bouncing" or not making good contact with the comm. If you have fitted a weaker spring on the negative side then the brush may be bouncing.. Check the condition and wear on the brush compared to the other side...... These are just some suggestions.
I am no expert but I would have a look at these possibilities.

Steevo

popsracer

How long has it been since the Com was cut? An out of round Com will "Spark" and cause glitching (besides bad performance).
It might be a good idea to replace the brushes at this time also.

zoombie

have no idea how long since the comm was last cut as it was a second hand motor...

sosidge

A small amount of sparking is normal, a large amount means it's time for a clean and rebuild.