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  1. #1
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    Shimano XT M785 Caliper Mishap

    Hey all,

    I have the XT M785 brakes as the thing keeping me from flying into that tree at a high rate of speed, and have a quick repair/maintenance question I couldn't find an answer to in searching or Shimano documentation.

    Managed to make a rookie mistake in bleeding front brake. My shim between the pistons slipped and one of the pistons got cocked on an angle, with fluid seeping out where the piston is farther out.

    1) Any idea if this can be remedied? i.e. dismantling the caliper and re-seating everything?
    2) Can I even dismantle that caliper? I struggled to find a way last night short of pulling the piston right out.
    3) Are the seals compromised, or will they be if I pull the piston completely out?

    Any knowledge and wisdom is much appreciated. All the bike shops around me look at me funny when I say hydraulic disc brakes...
    I like mountains. I like seas. I do not like mountainous seas.

  2. #2
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    I did the same thing a while back. Just pushed the piston back in gently, worked it in and out a few times to make sure it was running smoothly and the seal was properly seated, then bled the whole system - there was a lot of air in there!

  3. #3
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    Good to hear! I'm definitely expecting a ton of air, but I'll gladly deal with that over having to replace parts or whole calipers...
    I like mountains. I like seas. I do not like mountainous seas.

  4. #4
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    After my 'mishap' I made sure I bought one of the Shimano yellow bleed spacers so I didn't have that problem again. At about five bucks its worth it, as I wasted about that in oil!

  5. #5
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    Well, I figured out why it was leaking... when the piston cocked inside the caliper as the shim slipped it looks like it at the very least cracked the ceramic piston.

    I might have done the rest of the damage when I, in a panicked moment, referred back to automotive brake memories and used more force to try and get the piston back in. Instead, I should have referred back to my material properties courses to recall the brittleness of ceramics...

    Now shopping for replacement piston, which doesn't seem to exist. So maybe the M785 caliper for sale at Jenson? Those heat sink fins look fancy.

    I like mountains. I like seas. I do not like mountainous seas.

  6. #6
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    Ouch, that sucks!

    You should know that Shimano don't sell spare parts, the only "official" way to get a new piston is to send the brakes in for repair. If you feel brave and have some extra time and cash on your hands, you could try to replace the pistons with spares from another manufacturer. The 22mm piston size used by the XTs is fairly common and several other brakes, like Formula R1s and Hope X2s use pistons of the same diameter. The Hope X2 phenolic pistons, which should match original ceramic ones in both heat conductivity and diameter, will set you back about $10 for a pair.

  7. #7
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    Wow! That's a warning to all of us about how delicate those ceramic pistons are. Come to think of it, when I had the same problem it was on a Deore, which has plastic? pistons, which can obviously take more abuse. I would go new caliper as I wouldn't be confident at getting all those small bits of gritty ceramic out of the system!

    Definitely use the proper Shimano yellow bleed block in future!

  8. #8
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    Yeah, went with the new version of the M785 caliper, and ordered the yellow bleed block as well!
    Yeah, ceramic is way harder than the plastic or metal ones (depending on the metal), but way more brittle. Just need to be careful, lesson learned.

    Now hopefully I don't do anything else stupid for a while, I can feel the wallet glaring at me from here!
    I like mountains. I like seas. I do not like mountainous seas.

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