Shimano XT M775 Experts: Dominant Vs. Sticky Piston- Mtbr.com
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  1. #1
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    Shimano XT M775 Experts: Dominant Vs. Sticky Piston

    I've got a nearly new set of XT 775's on a new bike, having a problem with one piston that will not fully retract. have gone through all the steps, cleaned the piston with alchol, lubed with mineral oil, did a bleed, reset the pistons, manual centering, floating centering with mounting bolts loosened, etc. etc. I still have one piston (closest to the hydro hose fitting) that will not retract all the way in. it runs rub free in the stand, but tends to kiss the rotor occasionally at speed, in turns. i have a feeling this will get worse as the weather cools, and the piston seals get stiffer.

    is this the normal "one piston is more dominant than the other", or do i have a truly sticky piston?

    this is my third set of m775's on a given ride. i've had to warranty a new caliper before for the same problem, but it was much more obvious. the remainder of the calipers i've used have had total retraction of the pistons on release of the lever.

  2. #2
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    You probably have a....

    dominant piston. The fix is actually quite easy, and can be found in the thread that covers "centralizing" your caliper here, http://forums.mtbr.com/showthread.php?t=565754 , or you can go directly to the Hope website which is where the video originated here, http://www.hopegb.com/page_mep_force_38.html . It's really quite a simple process and any brakes that I have tried it on show a marked inprovement in modulation and overall performance.

    What causes a dominant piston is quite simple. At some point in time in the life of the brakes one piston ended up with more fluid behind it than the other. Whether the lever was pulled slightly with one lever supported and the other free to move or whatever is imaterial. The way fluid flows into the caliper is the reason this can happen. There is no metering system between the two sides of the caliper. The fluid simply flows down a single line and the it is split internally between the two sides. If one side is hled stationary and the other if free to move, fluid only flows to the piston that is capable of moving. When the lever is released it still pulls fluid from both sides, but the stantionary side retracts fully and the now dominant side doesn't because of the extra fluid volume behind it. You now have one side that contacts the rotor early and one that contacts the rotor late. Flex in the rotor won't allow the situation to correct itslef, so you have to.

    In the instructions for centralizing your caliper you first center you caliper not with the pads but with the caliper body. And then adjust the pads. The method is quite simple and easy to do. So take a look at the video and go from there.

    If that doesn't work then you have other problems.

    Good Dirt
    "I do whatever my Rice Cripsies tell me to!"

  3. #3
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    hah!

    while i was waiting for someone to respond, i found that exact post. looks like its working, at least in the workstand.

    thanks for the reply, good synopsis of the hows/whys.

  4. #4
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    I also had this problem once and this was a problem for me. Never found that post to fix it, LOL

    I fixed it in another way.

    I took the caliper of and took the brakepads out. I found that one piston was almaost stuck and did not wan`t to move before the first piston had "locked" against the rotor and started to press the rotor out of the centerline. What i did was i lubricated the piston with a very thin oil around the outside of the piston. I then moved the piston in oand out and it made the piston loosen up and move freely. Problem solved for me.
    Giant Anthem X, 23 lbs and dropping....

  5. #5
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    for sake of argument, how does this fit into things?

    http://bike.shimano.com/publish/cont...0Procedure.pdf

  6. #6
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    Quote Originally Posted by cjn1014
    for sake of argument, how does this fit into things?

    http://bike.shimano.com/publish/cont...0Procedure.pdf

    Yeah I use chain lube, and I center the caliper eyeballing it to the rotor...

    key point clean the pistons....

  7. #7
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    Quote Originally Posted by cjn1014
    for sake of argument, how does this fit into things?]
    Good advice. For mineral oil brakes (Shimano/Magura), mineral brake fluid is good for cleaning/lubing the seals. For DOT brakes, it would be DOT fluid or silicone lube. Chain lube is probably not a good idea as it could easily contain substances which would damage the piston seals.

    What use is a philosopher who doesn't hurt anybody's feelings? -
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  8. #8
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    For some reason I can't view the video... anyone know if it's still up on Hope's website?
    Is there a posted written summary anywhere?

  9. #9
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    Quote Originally Posted by uwprunner
    For some reason I can't view the video... anyone know if it's still up on Hope's website?
    Is there a posted written summary anywhere?
    Do you see this...

    <embed src="http://blip.tv/play/rQTgnCQC" type="application/x-shockwave-flash" width="480" height="341" allowscriptaccess="always" allowfullscreen="true"></embed>

    What use is a philosopher who doesn't hurt anybody's feelings? -
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  10. #10
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    Nope, just a blank white box.
    My computer is telling me I have flash player installed....

    "You have version 10,0,22,87 installed"

  11. #11
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    Can't you just update your flash player?

    Here's the YouTube version...

    <object width="560" height="340"><param name="movie" value="http://www.youtube.com/v/akIWGqEE0PY&hl=en_GB&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/akIWGqEE0PY&hl=en_GB&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="560" height="340"></embed></object>

    What use is a philosopher who doesn't hurt anybody's feelings? -
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  12. #12
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    Excellent. Thanks Steve!

  13. #13
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    Thank you this thread allowed to fix my rear brake. !

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