I agree with the above but would add this:

If the rim is bent, it could be because of a serious pothole or because it's worn out. If the rim is bent towards the outside, check the thickness of the rim sides. If they seem to cave in rather than being straight, you might have rims that are worn off because you used them a lot. That's only a problem on a well-used bike that could have, let's say 10 - 20 000 km depending on the amount of braking you do, weather and terrain conditions.

Another possibility is either grease or sticky stuff at one point which would make the brakes grab less or more at that point.


Unless you feel a serious bump or notch in the rim, I would leave it there. I think it is better to under-correct the problem and try it then to over-correct it. You can't "un-file" a rim, and aluminium doesn't like to be bent over and over.