In reply to Byronius Maximus:
> It's not a magic cure (and maybe you've tried it already) but disc brake degreaser is worth a shot....Iso propyl alchohol does a similar job. The above might just be that, I'm not sure.
Certainly the aerosol brake cleaner I bought for my motorbike smells just like isoprop. However, it might contain other stuff as well - the label doesn't say.
> I'm not so sure about using WD40/GT85 as I have accidentally sprayed my discs with that (over spray while cleaning the frame) and that has caused noise rather than curing it.
WD40 is basically paraffin i.e. mineral oil, and GT85 is a lubricant containing PTFE (made by the same company as WD40, incidentally). Why anyone would even consider spraying their brake pads or discs with lubricants is beyond my comprehension.
> Giving your pads a light sanding to get rid of the top layer containing any crud might help. Basically, getting rid of contaminants is the key really.
My front brake is normally nice and quiet but the recent excessive quantities of snow, rain and crud did start it squealing. I actually took the (sintered) pads out yesterday and gave them, the disc and the caliper including the pistons a good scrub with isoprop-soaked cotton buds. Took it out for a run today - still cold, wet & clarty - and not a squeak. (I had sanded the pads with wet&dry a few days previously but that clearly wasn't sufficient.)
I don't use grease or anti-seize between the pistons and the pads. I'm even a bit chary about using mineral oil to lubricate the pistons, as is often recommended (even on this Park Tool video: ). Basically, I like to keep lubricants away from my brakes as much as possible.
> I had badly squealing brakes and a combination of the above seemed to work for the rear, though it really seems pot luck. The front one turned out to be because the seal on the front caliper was leaking so getting mineral oil on the disc and caliper - the noise was horrendous! I only realised that was the issue when I had almost no braking. That was on a 105 caliper and apparently this isn't completely unheard of on these, so worth checking for that if you have Shimano brakes, before you suddenly have no brakes when you need them!
Shimano brakes do have a bit of a reputation for developing leaks in the piston seals. I don't know whether they're worse for it than other brands but it's not helped by the fact that they don't sell replacement seal kits like e.g. Hope do (though other opinions seem to suggest that Hope need to do this because they're significantly more prone to blowing seals than Shimano; I have no opinion either way). The most likely reason for piston seals leaking is erosion of the square profile O-ring due to dirt being pushed past it when the piston is pushed back in e.g. when fitting new pads, or bleeding the brake. The takeaway from that is always to clean your brakes thoroughly before working on them. (In fact, of course, that should be the mantra for all workshop tasks.)