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Sorry for the late reply! Yes, it does sound like they're contaminated, but I have no clue how it could have happened.
Can dust contaminate pads? That's the only theory I have given the circumstances. Both times this happened, my bikes were sitting in my living room in a dusty and humid apartment. If I have to replace a 40€ set of pads every time my bike sits on the trainer for more than a month, it's gonna become expensive really quickly. A rim brake frame seems tempting but alas I need 30mm tyres and mudguards.
It is not a leaky piston issue as both front and rear got the same issue, though I'll double-check that there's no oil on the calipers.
For cleaning, do you think isopropyl alcohol on the rotors will suffice? I'm worried I might not clean the rotors well enough and they might re-contaminate the new pads.
Veblen-
do you think isopropyl alcohol on the rotors will suffice?
Yeah, but also sand them between IPA cleans, I spin the wheel and hold sandpaper/emery cloth to them as they spin, watch your fingers.
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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.
snottyotter
Howard
Sounds like they're contaminated, this could be from you or there's a chance of a leaky piston if you're sure it's not from you. You can try embracing the squeal and get it really hot braking down big hills but it's an unlikely and temporary solution, sorted my front out for a ride though. Sanding and cleaning the pads and rotor may help, otherwise it's clean rotor and new pads and if it returns after being really careful, new caliper.