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  1. #1
    mtbr member
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    XT 4 pots all of a sudden get weak.

    Running old school 755 XTs (four piston calipers) and they've always had great strength.
    One finger stopping.
    All of a sudden, on a ride, the rear break goes weak, very weak to the point where it won't lock up the rear tire.
    Right before that I had just hit it the rear brake pretty hard. (hit, as in used, not impacted)

    Lever is fine, holds pressure fine, everything feels same as always, just no stopping power.

    I pulled the pads and sanded off any glaze. No change.
    Bled it just to make sure. Nothing.
    Put an old set of decent shape pads I had laying around. Still no good.

    Ideas?
    Last edited by roxtar; 09-08-2009 at 09:38 AM.

  2. #2
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    All (4) the pistons move when the lever is pulled right? Try cleaning the rotor or swapping that out.

  3. #3
    fliernh
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    Check and make sure that the pistons are free to move on both sides. Sometimes one side will seize and not be moving or will be stuck on the seal on one side and retract too much causing stroke to run out before good engagement. Not sure if I described that well but on some brakes, one side stops moving or sticks to the seal. Way to fix or check this is to push pistons all the way back in and cycle a number of times with an appropriate size spacer (with pads in place) to make sure both sides are moving fully. push both sides all the way back in and reinstall wheel.

    This has worked on a couple different types of brake though I have not done it on the specific brake you have. Can't hurt and it is free, Good luck.
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  4. #4
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    Contaminated pads?

  5. #5
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    Clutching at straws here. Have you checked the hose for bulges? Maybe the inner hose has kinked or ruptured? Assuming you have the same brake on the front you can try connecting the rear hose to the front lever and see if it is a problem with the master cylinder.

    If it is still a problem can you rig it to run using the front hose on the front wheel with a different spacer or rotor size?

    Tim

  6. #6
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    Forgot to mention that.
    I did check and all 4 pistons are moving.
    Will look at brake line next.

  7. #7
    ballbuster
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    inspect the pads

    Quote Originally Posted by roxtar
    Running old school 755 XTs (four piston calipers) and they've always had great strength.
    One finger stopping.
    All of a sudden, on a ride, the rear break goes weak, very weak to the point where it won't lock up the rear tire.
    Right before that I had just hit it the rear brake pretty hard. (hit, as in used, not impacted)

    Lever is fine, holds pressure fine, everything feels same as always, just no stopping power.

    I pulled the pads and sanded off any glaze. No change.
    Bled it just to make sure. Nothing.
    Put an old set of decent shape pads I had laying around. Still no good.

    Ideas?
    Make sure the pads have not worn a groove, and maybe the tops are touching. This happens when the rotor is too small, or the brake caliper is mounted too high (like the brake tabs weren't located in the right place). I saw this happen once when somebody used a 160mm rotor on a brake meant to use a 165.mm. The pads left a little tiny line of unworn pad where it reached over the top of the rotor, and eventually they met in the middle and the brake stopped working.

  8. #8
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    Well, no luck anywhere here.
    Checked the line (stainless steel) by sliding fingers down it while pumping the brakes. All feels kosher; no bulges.
    First thing I did was to sand the pads down to get rid of any glaze so that took down any high spots there might have been.

    I'm at a loss.
    I guess next step is to try new pads.
    Hoping these are still available.

  9. #9
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    You should have no problems finding these pads.

  10. #10
    mtbr member
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    OK, lots of choices for pads.

    ECB Grimeca sys 8
    ECB Grimeca sys 12
    Shimano metal
    Shimano resin
    Cool Stop organics

    Recommendations?
    Last edited by roxtar; 09-08-2009 at 10:06 AM.

  11. #11
    Ultimated Outsider
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    I used Goodridge G+. I also like the Shimano resin because they are quiet. Don't buy the Grimeca Sys 12 pads because they are to big. Sys 8 is cool and so are the Hope Mono M4 pads.

    You might want to also check the rotor and make sure is not contaminated with oil or worn. Swap it with the front to see if it work.
    If I disagree with you, it's because you are wrong.

  12. #12
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    Tough to swap, running a 203mm on the front.

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