i've decided that most of that stuff is coming off the rim due to the machining of the braking surface. they do a pretty coarse machining job and the ridges start coming off and imbed in the brake shoes after a few hundred miles. i pry it out with a knife if it refuses to work it's way out on it's own or before my patience is exhausted. it may happen a few times then they all wear off enough that it never happens again. only had it happen on machined braking surfaces.
Last edited by hueyhoolihan; 05-01-14 at 02:20 PM.