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  1. #1
    el Rappazapator
    Reputation: joschi's Avatar
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    Bonking ... not feelin' well Problem with XTR rear calliper on Scalpel

    I've got a problem with my XTR rear calliper on my scalpel. While riding the pistons seem to retract automatically back into the calliper. So I've to pump first a view times before the brake grips. If you're prepared for this it's no big deal, but in critical situations it's often quite dangerous.

    Actually I've got this calliper for nearby two years. It works very smooth and mostly both pistons act equally so there is not much problem with dragging. The seals seem to be tight and there is no air in the system.

    I think the seals are somehow a little worn, so it may take not this much effort to push the pistons back into the calliper. Because of the flexible rear construction of the scalpels rear the rear wheel always "works" a little during the ride. So the disk may sometimes touch the brake pads and pushing them a little aside.

    Has anyone a better explanation for this behaviour?
    Is there another solution then replace the whole calliper?
    If I have to replace the calliper, do you recommend the use of steel flex hose, the original Shimano hose seems to be a little flexy.

    Thanks for any help and advice!

  2. #2
    Trail rider and racer
    Reputation: Trevor!'s Avatar
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    Have you tried a rebleed?
    What about replacing the seal that you think might be worn?

    Then
    If that fails perhaps send it into Shimano to see what the trouble is?

    My history with my XTRs was strictly stuck pistons the absolute opposite of this!
    Trev!

  3. #3
    All Lefty's, all the time Moderator
    Reputation: MendonCycleSmith's Avatar
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    Quote Originally Posted by joschi
    I've got a problem with my XTR rear calliper on my scalpel. While riding the pistons seem to retract automatically back into the calliper. So I've to pump first a view times before the brake grips. If you're prepared for this it's no big deal, but in critical situations it's often quite dangerous.

    Actually I've got this calliper for nearby two years. It works very smooth and mostly both pistons act equally so there is not much problem with dragging. The seals seem to be tight and there is no air in the system.

    I think the seals are somehow a little worn, so it may take not this much effort to push the pistons back into the calliper. Because of the flexible rear construction of the scalpels rear the rear wheel always "works" a little during the ride. So the disk may sometimes touch the brake pads and pushing them a little aside.

    Has anyone a better explanation for this behaviour?
    Is there another solution then replace the whole calliper?
    If I have to replace the calliper, do you recommend the use of steel flex hose, the original Shimano hose seems to be a little flexy.

    Thanks for any help and advice!
    My thinking is that there is a check valve that creates the "auto adjust" feature of hydraulic brakes that allows the pads distance to remain constant throughout it's wear. That said, it would seem yours is not "checking". I am remiss to tell you whether it is in the Lever or the caliper, but that seems to be where to look. Try pushing the pistons back in the front, then in the rear, is the rear much easier? I doubt that seal wear would make them walk back inward. That's an odd one for sure!

  4. #4
    LA CHČVRE
    Reputation: Dan Gerous's Avatar
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    It's actually the seal flex that determine the 'travel' the piston have to go back in after they are released, ie, the 'auto-adjust' feature. It's very probable that the seals are at fault here. They might be worn out (by natural causes and/or by artificial causes like a strong chemical that don't belong there like degreaser)...

    Have you checked the brake forum for similar issues?
    Last edited by Dan Gerous; 11-02-2005 at 07:04 AM.

    DAN.GEROUS.NET : MOUNTAIN BIKING : CYCLOCROSS : ROAD :

  5. #5
    mtbr member
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    Quote Originally Posted by Dan Gerous
    It's actually the seal flex that determine the 'travel' the piston have to go back in after they are released, ie, the 'auto-adjust' feature. It's very probable that the seals are at fault here. They might be worn out (by natural causes and/or by artificial causes like a strong chemical that don't belong there like degreaser)...

    Have you checked the brake forum for similar issues?


    Thats correct, the 2 piston calipers use a "roll back" seal to provide the return action of the piston.

    Sounds to me like they've worn a bit. A quick check for residual oil on the piston and on th back of the pads willconfirm this cos those calipers don;t leak.

  6. #6
    el Rappazapator
    Reputation: joschi's Avatar
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    We the people ... Problem reproduced by hand

    Well, thank you all for your advice and tips. I would like to answer all your questions as good as possible, so here we go:

    I've bleeded the system several times, also different LBS technicians expected air to be the problem and bleeded the system. Still the problem persists…

    As I know, Shimano will not sell seals kits for their callipers, just complete units.
    Besides this, the removal of the pistons is a real pain in the a***

    As I understood the "automatic wear adjust" of disk brakes it is realized by a special geometry of the piston seals. So there should be no check valve or something like that in the system.
    I don't know that much about the Shimano system, but I know for sure that Hayes uses slightly angled seals to do the auto adjust thing.

    By retracting the pistons by hand it isn't a big difference between the front and rear calliper. Maybe the rear calliper is a little easier, but just slightly.
    I think the problem doesn’t occur in front because the wheel fixation on the Lefty is much stiffer the in the rear so the front wheel won't wobble this much during a ride.
    How ever, the Shimanos glide back quite easily compared to the Hayes callipers on my Jekyll.

    After bleeding the system I sometimes have a little bit of oil around the hydraulic oil reservoir on the lever. But I think this is quite normal, I've to wipe it a view times and after two or three days the problem is gone. Maybe I have to replace the seal membrane and plastic top cap.

    And yes, I'm guilty - I use Acetone on a regular base to clean my callipers, pads and disks. How ever, I do the same thing on my front calliper without any negative effect.
    By the way, with Acetone you can degrease pads that have been completely soaked with oil and they will work flawless again. In reward your workshop will smell like hell…

    ************************************************** ************************************************** ***

    Actually, just a view minutes ago, I've reproduced the retraction effect by hand. I fixed the bike by holding it on the saddle with one hand and then shake the rear wheel a view times with the other hand. After that procedure I had to pump two or three times to have the brake pads touching the disk. So the cause of my problem is found and reproducible. I think it’s the combination of the very flexible back of the Scalpel and worn seals and pistons.

    Once more the XTR stuff has worn out quite quick. I've replaced XTR parts for about 1000$ during the last two years. This stuff really looks great and is very light weighted but in return it wears quite quickly.
    I've already replaced two of these wonderful titanium cassettes, two sets of chain rings, numerous chains and three bottom bracket bearing units. And now it looks like replacing a calliper. Quite an expensive hobby…

    By replacing the calliper I think about also replacing the original hose with some steel flex stuff. Has anyone changed from original Shimano hose to steel flex hose? Is there a real difference or is it just hype?

    Happy trails and thanks for all!

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