I recently acquired a mid-1980s Saracen MTB, equipped with cantilever brakes. I tested it out for the first time yesterday, and the front brake made a scraping noise every time I applied it. The brake pads are fairly new, but on close inspection, they seemed to have bits of metal stuck in them. I picked at the bits with a pin, and they came out fairly easily, leaving pits in the rubber. The metal didn't come out as large pieces, it sort of disintegrated into powder. I assume the metal comes from the rim, but what's causing it to collect in certain places on the shoes? Is it that the rubber has pits in the surface already, which happen to collect the metal as it wears off? If not, what's happening?
I assume these bits of metal accelerate rim wear, so I'm thinking I'll check the pads occasionally in case it happens again, is there anything else I can do to deal with it?
Thanks in advance.