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  1. #1
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    sticky piston in XT 2008?---Help a newbie please!

    Hi...

    I was looking that at my rear brake, one piston was coming further than the other...After reading some posts here suggesting to use mineral fluid arround the piston area (without the brake pads ) and that will cure the problem, well I don`t have mineral oil right now but I did use a little of isopropyl alcohol with a Q-tip to clean all the piston area, just press the lever till they come out a little and clean the surrounding area with it...

    Could this alcohol kill the seals of the pistons?...Is that safe?...Also I think that it didn`t fix the problem , because I`m still having the issue just that a little bit better...I mean, less sticky piston...

    Other thing to note was that when I was pushing one piston out suddenly there was like a "pssssttt" it was really fast, what could that be?...A air bubble going IN the system or getting OUT the caliper?

    I had to depress the lever like 20 times to get back to the original feeling and now they are like they were before...so I think that there`s no air inside...

    Could that be possible? that some air goes into the system, trough a "too further out" piston?...

    Also, what happens if I depress the lever until both pistons come out?..do they brake? do they go out of the caliper and I`ll be with a damaged brake?

    thanks a lot, and sorry for so many questions, but I`m a total newbie on this and most of the question don`t have answer on the disc brake FAQ...

    thanks!
    bye

  2. #2
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    Iso alcohol helps clean but I think it dries out the seals a bit. I had the same problem with my XTRs and they are being worked on by Shimano right now. Shimano sucks when it comes to customer service, by the way.

    However, call them and mention this to them. If they say it never happens, tell them about a guy who (three weeks ago) sent his brakes in to get them worked on. The problem is somewhat frequent with XTs/XTRs. I think you have to use a bit of mineral oil after the cleaning or the problem comes back frequently. Do call them- a bit of customer feedback will only help us in the long run.

    You probably had a bit of air enter the system and may need a brake bleed eventually. Also, if your pistons fall out, you will have to do a full bleed due to tons of air in the system so be careful. If you simply push them back in, they will not work.

  3. #3
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    How do I know if some air got into the system? The lever feels fine...I will buy the mineral oil tomorrow to so the same I did with the alcohol...to lube everything...
    BTW I could push the pistons back in with finger force...no tire levers or other stuff needed...is that normal?...

    thanks!!

  4. #4
    wyrd bi ful rd
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    If you have air in the system the levers will feel spongy. Do not use any alcohol based liquid to clean the seals as alcohol only dries them up as flyer mentioned. Just use mineral oil as that is what is in the system on the other side of the seal anyway.

    Bleeding the brakes is just a simple process. Just follow the instructions to the letter.
    Last edited by chinaman; 01-05-2008 at 10:43 AM.

  5. #5
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    chinaman has it right on. When cleaning, don't push the pistons out too far- you can even use a block to stop that from happening- any biek shop will have those piston blocks.

    If air is in your system, the brakes will feel spongy and less effective. If it feels normal, it may be fine, at least for now. Pushing the pistons back is easy to do with your fingers but 99% of the time, will result in a brake bleeed so use a block.

  6. #6
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    what do you mean with : but 99% of the time, will result in a brake bleeed so use a block.
    And why it will be fine "at least for now"...isn`t that firm feeling on the lever, constant?

    Thanks a lot...sorry for the need of clarification but english is not my mother language...

  7. #7
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    Well, if the air bubble becomes bigger, you will feel sponginess and will have to bleed the brakes.

    A block acts like a disc rotor and prevents the pistons from falling out. With a block, you can clean/lube the pistons and not worry about the pistons falling out. They may not be called "blocks" but are small rectangular pieces of plastic that can be wedged in between the pistons like like the disc rotor is when the wheel is on.

  8. #8
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    ok now I understand...well I`ll have to wait...when could it be become bigger? when the brakes get too hot?...

    I have two kind of "blocks", one that is like yellow that goes all the way into the caliper, very thick...and other red that engages at the brake pad retainer and acts like a rotor...But could I depress the pistons all the way out (without the brake pads in) and they will hit the block?...I mean that block is the limit of the pistons?

    thanks!

  9. #9
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    Temperature/pressure changes can cause the air to expand but if it's fine now, it should be okay. The thin block will keep the pistons from falling out. It isn't the max that the pistons can extend but it's very close and will be enough to get them cleaned up and seals lubed a bit.

    Keep an eye on the pistons- you just need them to extend out a couple of mm to clean them properly.

  10. #10
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    Yup the yellow one you use with the pads out the red with the pads in.

    I get the sticky pistons as well (XTRs), mostly in the winter with snow and salt but sometimes in the summer.

    If I am in a hurry I just loosen of the clamp bolts and set the caliper so it doesn't drag when the brakes arn't on. Works pretty good. (just let the system find its own center).

    But sooner or later I take out the pads (don't bother with the blocks) and move the pistons out with the lever, and push them back in with a tool of some sort.

    If one piston doesn't move I hold the moving one with a pair of pliers, and that will force the other to move, then I just put chain lube on the pistons and move them both in and out a few times.

    Put it all back together and away I go. I gave up trying to lube the pistons with the pads in cause I spill oil on the pads too often.

  11. #11
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    I don`t that you could use chain lube for that matter...is that recommendable...??. Also doing some research to see if the isopropyl alcohol could be good I found this:

    http://bike.shimano.com/publish/cont...istons.pdf.pdf

    Read at the end...
    thanks!

    BTW, should both piston protude EQUALLY...I mean there`s always one that protudes at least 2-3mm more than the other...is that a "normal" range of tolerance?

  12. #12
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    2 to 3 mm a little less than an 1/8 of an inch.

    When they are good they are pretty much bang on equal.

    Iso propyl alcohol is a very good cleaning agent for brakes and brake pads.

    But if you don't lube the pistons they will get sticky very quickly.

    I use an oil based teflon lube, works fine.

    I don't normally need the alcohol at all I just use soap and water with a good rinse.

    But I have a bottle of iso propyl in case.

  13. #13
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    ok, thanks for the message. BTW I don`t get any rubbin at all...
    Later I will post pics about them so people can judge more accurately...

  14. #14
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    I have used windex in the past to help lube the pistons without drying out the seals. works like a charm........

  15. #15
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    I only just saw this thread now and thought that I would add the following information. It is directly from Shimano XTR service instructions but is the same for XT. Be advised that it is possible that one piston sticks out more because there is too much mineral oil in the system.

     Adjustment when the pistons are not operating correctly
    The caliper mechanism includes two pistons. If these pistons do not operate properly or if
    they protrude unevenly, or if the brake pads remain in contact with the rotor, adjust the
    pistons by the following procedure.
    1. Remove the wheel and the brake pads.
    Clean the pistons and surrounding area, and remove the reservoir tank cover.
    2. Push the piston back in straight, without bending it. Note that some oil may overflow from
    the reservoir tank at this time.
    3. Install the brake pads and the pad spacers FB (red).
    4. Depress the brake lever as far as it will go, and then operate it several more times so
    that the two pistons all move to their initial positions.
    5. Remove the pad spacers, install the wheel, and then check that there is no interference
    between rotor and the calipers. If they are touching, adjust using shims.
    6. After checking the oil level, replace the reservoir tank cover.

    Ronnie.
    The trouble with having an open mind is that people will insist on trying to put things in it.

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