You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
The GF's rear brake was playing up - needed a bleed (again? a 6 month old XT brake needing a second bleed within 3 months?) so I bled it.
Went out at the weekend and after a decent ride the rear brake was playing up again. When I had a closer look, one of the pistons won't be pushed back in to the caliper - it sounds almost sharp or gritty. It will pump a bit with the lever but won't retract back.
I pumped the pistons out a bit and tried to push fresh fluid through but got no improvement.
If I take the caliper apart (involves the need to purchase a (T30?) torx driver) to clean it, what are my chances of getting it back together?
Alternatively, whare will sell a XT M785 caliper? Can't find any on rosebikes, at CRC it's about £45.
Its actually quite easy to take a Shimano calliper apart and re-assemble - my thread [url= http://forums.mtbr.com/brake-time/ever-wondered-how-hydraulic-brake-calliper-works-848452.html ]here[/url] may help (and yes, you'll need a T30). Trouble is, if you find you need to do more than clean, spare parts aren't available. Grittiness could just be grit between piston and bore, or could be more serious in that the ceramic piston itself is damaged. They're quite fragile and I've seen ones disintegrate if you use something sharp to push them back in, or don't put even pressure on them. (note: Internally, the M785 calliper is identical to M666 shown in my thread)
6 month old XT brake
Warranty? at least 12 months
My guess (from the word gritty) is that your piston is fractured.
Time for a new calliper. But take it back under warranty and see what they say.
perth - thanks. Gives me a bit of confidence.
mikewsmith - you're probably right, but I reckon it's just as likely that I introduced contaminants when bleeding - not a failure. So then we'd have to pay labour rates... We also want to get this fixed so we can go riding rather than wait for it at the shop.
piston is fractured.
Could explain why it's needed so much more attention than any of my other shimano brakes..
Full marks go to [b]shortcut[/b]! 
😥
Yes - that's the underside of the piston that's fractured - definitely no damage from pushing pistons wiht my big plastic tyre levers 8)
[img] 
[/img]
Now to really see how good the LBS's after-sales is..
Trouble is, that can also happen by pushing them back in crooked, even with a plastic tyre lever. So, yes - you're about to find out how good the after-sales service at your LBS is, by whether they argue the toss, or just give you a new one 🙂
Thank you!!
If you get no luck - possible! The pistons are not available separately.
But the pistons are the same as you get in an SLX calliper so get one of those and use the piston from there.
Anyone would think I have broken pistons before reading this. And you would be right. Shimano brakes are excellent but very delicate and easy for the ham fisted to break.