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I've not ridden much over the last 18 months for health reasons and my bike has been in the shed and unused since early November due to another heart attack.
I got it out a few days ago and discovered that I had no braking power from the front brake (it felt like the pads were made of glass). Even if I clamped the brake on as hard as I could, I could still turn the front wheel by standing on a crank. There was a spot of honking that cleared up quickly; lever pull felt normal (I'd assume with a leak it wouldn't).
Cleaning the rotor didn't help so I'm assuming that the pads (sintered) are contaminated. I found an old set of part worn resin pads and popped them in and, bingo, braking's back to normal. I'm hoping that I don't have a leaky Shimano calliper (SLX M7100).
The 'contaminated' pads seemed completely dry (covered in braking material dust as were the insides of the callipers) and I could see no oil marking on kitchen towel; no signs of oil on the rotors either nor could I see any beading of oil around the pistons when I applied the brakes hard with a spacer block in.
I understand that leaks are small but I'd be surprised not to see some sign of oil with enough contamination to render the brakes practically useless. I'd also assumed that faulty Shimanos leak when brakes are applied and the pressure increases rather than just seeping out of a system unused for 4 months.
I suppose that it's also not improbable that GT85 has found its way onto the pads/rotor as a result of maintenance in an enclosed space.
Any other thoughts? I guess that all I can do is ride the old resin pads and see if they become contaminated too (I've still got the old M675 callipers I could refit).
rather than just seeping out of a system unused for 4 months.
Various generations of Shimano brake calipers have been a bit leaky. I've put a bike in the shed, working fine, came back and the brakes didn't work anymore.
You might find some oil on the back of the pads.
I have some others, same generation, which are still fine after years of neglect, so YMMV.
If it happens regularly, and they are still in warranty, you can return them.
You might get away with cleaning the rotors and baking the pads in the oven.
No sign of oil on the pads anywhere but I'll keep an eye out and see if the current pads become contaminated with use.
I'm hoping that it's just a one off, but who knows?
If I've had a bike unused for a long time I'll often experience the same symptoms. It used to be an annual thing with my fatbike. Bake the pads in the oven and they're good as new.
I think it's just contamination and lack of use as, once done, they're fine.
If it is the leaky shimano caliper issue then the fluid was not visible any of the multiple times I've lost a caliper to it.
I had similar with some deore calipers - if I left them for any period of time the brakes lost power. Sometimes if you got them really hot it would help but it would come back again. Got rid and haven’t had a Shimano brake on an mtb again - and I’ve never had the issue again. I think if used regularly the general trend is the Shimano calipers don’t then tend to do this - they don’t like being left unused.
If it’s in warranty I’d try to use that for new ones and then hope for the best that you get a non-leaky set.
I have the same with my old xt (785 I think) brakes. There's never any sign of oil leaking