• Something in your home has got on them. Dust / some kind of cleaning fluid / lubricant of some kind.

    Few of these things can survive the temperature generated by a steep descent with brakes on full whack. Repeat this until they are smoking.

    FWIW, Shimano brakes are shite, in that the pistons have a habit of fucking themselves and allowing small but ruinous amounts of fluid to escape. It could simply be both being fucked.

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