XT 775 caliper with sticky piston??- Mtbr.com
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  1. #1
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    XT 775 caliper with sticky piston??

    The pistons on the rear caliper do not come out equally and there is not very much pad clearance with the rotor. The pads do not retract as much as the fronts, and the lever is very firm.

    These brakes are new to me (used set), but I'm not new to these brakes. My previous sets never had these issues or felt like this. The front is fine.

    Is this a sticky piston? And how do I correct it?

    TIA
    Last edited by S-Works; 11-25-2010 at 09:04 AM.
    Astigmatic Visionary

  2. #2
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    Yup....

    Wash the bike...

    Remove the wheel get a thingy and push the pads and pistons all the way out...(remove the pads)

    Then squeeze the pistons back in....you should see a line of dirt....clean that up with iso propyl alcohol...

    Continue pushing the pistons in and out, until they both move smoothly and equally.

    Sometimes you need a pair of pliers to hold the one the moves first so you can get the stuck one moving....

    Once everything is good add a tiny drop of lube to each piston...then push out and reinstall pads and wheels.

    Make sure the caliper is centered over the rotor....adjust so that it is.

    Go riding.

  3. #3
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    Quote Originally Posted by jeffscott
    Yup....

    Wash the bike...

    Remove the wheel get a thingy and push the pads and pistons all the way out...(remove the pads)

    Then squeeze the pistons back in....you should see a line of dirt....clean that up with iso propyl alcohol...

    Continue pushing the pistons in and out, until they both move smoothly and equally.

    Sometimes you need a pair of pliers to hold the one the moves first so you can get the stuck one moving....

    Once everything is good add a tiny drop of lube to each piston...then push out and reinstall pads and wheels.

    Make sure the caliper is centered over the rotor....adjust so that it is.

    Go riding.
    Thanks for the response. I will give it a try.

    Can the pistons come out too far? I assume the way you get the pistons out is by lever aciton.

    Thanks again!
    Astigmatic Visionary

  4. #4
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    Yup....just be careful it really isn't much of a problem.

  5. #5
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    Quote Originally Posted by jeffscott
    Yup....just be careful it really isn't much of a problem.
    Good Deal! I'm going to do this and then get a fresh bleed on them.
    Astigmatic Visionary

  6. #6
    g3h6o3
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    My GFs XTR had the same problem. Took the wheel off, pumped the lever, cleaned with isopropyl alcool, push back in, repeat a couple of times and then it was all good.
    Check out my SportTracks plugins for some training aid software.

  7. #7
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    Quote Originally Posted by PissedOffCil
    My GFs XTR had the same problem. Took the wheel off, pumped the lever, cleaned with isopropyl alcool, push back in, repeat a couple of times and then it was all good.
    I've done the procedure 4 times, it's better, but still not equal to the outer piston. Keep going? Or something else?

    I did bleed them and they feel alot better.
    Astigmatic Visionary

  8. #8
    g3h6o3
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    What did you clean?

    You need to clean the outside of the piston that sticks out. After a couple of iterations with alcohol, put a bit of mineral oil back on the outer side of the piston and push it back in.

    If it doesn't work, maybe your seals need to be changed.
    Check out my SportTracks plugins for some training aid software.

  9. #9
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    Can you get new seals for Shimano calipers?

    I thought there weren't any service kits for their hydros

  10. #10
    g3h6o3
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    I'm not sure, never got to that point.
    Check out my SportTracks plugins for some training aid software.

  11. #11
    EDR
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    You need to push those pistons out as far as possible, without it actually coming out of the caliper. If that happens you'll have a mess on your hands and will have to fill/bleed again. I have to clean mine once a year or so to avoid the sticky piston issue. Just try and get those pistons out further so you can clean them up with a q-tip and some IPA. Apply some mineral oil when your satisfied they're clean.

  12. #12
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    A fair number of people here have accidentally popped the pistons out and shoved them back in without any further issues. I panicked, so I didn’t even wipe the piston clean, which I sure will do next time if happens (not!). So yes you "can" pop them out. I won't promise that it works after that, tho.

  13. #13
    g3h6o3
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    It seems no seal kits are available.
    Check out my SportTracks plugins for some training aid software.

  14. #14
    EDR
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    Quote Originally Posted by hatake
    A fair number of people here have accidentally popped the pistons out and shoved them back in without any further issues. I panicked, so I didnít even wipe the piston clean, which I sure will do next time if happens (not!). So yes you "can" pop them out. I won't promise that it works after that, tho.

    Good info. I popped one out just a bit once and I panicked also. Can't remember if I re-bled or just added a bit more oil. Guess it's not the end of the world if that happens

  15. #15
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    Quote Originally Posted by eatdrinkride View Post
    Good info. I popped one out just a bit once and I panicked also. Can't remember if I re-bled or just added a bit more oil. Guess it's not the end of the world if that happens
    Model: XT M775's (about 2010).

    I just popped mine all the way out through squeezing the lever repeatedly and holding one side down. I then cleaned the pistons with a rag and dishwash detergent - rinsed and dried. Wasn't quite sure how to clean the caliper bore as its very fiddly and the caliper is a one piece. Sticking some object in there apart from an ultra clean rag didn't appeal all that much but i tried with the rag alone.

    I think the piston was sticky due to uneven wear. There was some small amount of wear (indicated by a black scuff mark) on both pistons close to where the pad contacts the piston but on the piston barrel. When I reinserted the pistons I rotated the pistons from where they originally were and lubed with mineral oil making sure to insert them evenly.

    I definitely have to re-bleed as half the oil came out. The first side (on the opposite side to where the hose enters) I popped first and didn't have quite as much oil leakage as the side with the hose entering the caliper.

    Eventually I added more oil and rebled the system. Brakes seem to be working fine now and after centering the caliper on the frame, the brake drag has disappeared and so has the nasty clunk that was evident when applying the brakes!

    ....... time will tell!


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