The problem is the composition of the pads that come with those particular brakes. I had the same problem with a Tektro R556 brake that had those same pads, and they totally tore up the the rim braking surface and left black streaks on the rim. The pads chipped and crumbled, creating a nasty black dust that permanently stained the light tan sidewalls of the tire (Panaracer Pasela TG). Those pads are supplied in the light gray holders, and I replaced them with pads from a different model Tektro brake (R530) that has black pad holders and apparently a different pad material that does not have this problem. Since I changed the pads, the problem has completely disappeared and braking action is just as strong as before.
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