It is mainly dust that builds up in the disc drums, and as above because the shoes are not used for stopping the car, that dust gets ingrained into the drum thus creating what looks like rust on the meeting face of the drum to the shoes, the outer edge does get rusted as in some of the photos above.
at least once a year it's worth removing the drums to clean them free of dust and also dress the face with some emery paper, wear mask at all times.
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The above could most likely have been dressed but i chose to replace, as i had only just bought the car.
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old drum.
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New drum
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as always maintain is the answer.