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  1. #1
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    SRAM Guide R piston imbalance

    Notice the rotor scrubbing the pads on the front. Then notice squealing, like brake fluid contaminated the pads. Bled brakes, still scrubbing. Pushed pistons back, let out a bit of fluid, still scrubbing. The pistons on one side appear more extended than the other. Bad piston seals? These brakes are 2 weeks old.
    The only easy day was yesterday.

  2. #2
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    Before you go warranty it, it may be worthwhile to just take the calipers apart and clean the pistons. I only say this because a warranty claim took me a while.

    They have instruction on taking it apart on their service manual


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  3. #3
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    Reset pitstons (push all the way in). Advance the pads against a pad spacer. Install caliper on bike with mounting bolts just loose, squeeze brake, incrementally and alternately tighten the mounting bolts. If rotor is centered between pads, good. If not, tweak manually.

    There is no mechanism in brake calipers that makes pistons move equally. All the pistons are connected in parallel and whichever happens to move the easiest is the one that's going to advance.
    Do the math.

  4. #4
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    Quote Originally Posted by Lone Rager View Post
    Reset pitstons (push all the way in). Advance the pads against a pad spacer. Install caliper on bike with mounting bolts just loose, squeeze brake, incrementally and alternately tighten the mounting bolts. If rotor is centered between pads, good. If not, tweak manually.

    There is no mechanism in brake calipers that makes pistons move equally. All the pistons are connected in parallel and whichever happens to move the easiest is the one that's going to advance.
    Doesn't work.
    The only easy day was yesterday.

  5. #5
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    If you can determine which of the four pistons is stuck, you can try to isolate it.

    You can use (or make) a custom bleed block to keep all but one piston fully retracted, allowing you to work the stuck piston in and out.

  6. #6
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    Op, did you find a solution? Same issue here. I believe that the piston imbalance is why my brakes howl. I've tried pumping each piston individually to clean and lubricate then with lithium grease but they still don't move evenly. This has been a problem since new.

  7. #7
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    Quote Originally Posted by Bike Therapy View Post
    Op, did you find a solution? Same issue here. I believe that the piston imbalance is why my brakes howl. I've tried pumping each piston individually to clean and lubricate then with lithium grease but they still don't move evenly. This has been a problem since new.
    LBS replaced the levers, which should solve the problem, I haven't checked they are sitting in a bag on my work bench. I replaced my brakes and rotors with XT's, now all is well. My Guide brakes howled and sounded like a pack of hyenas when braking. When I put the XT brakes on the AVID rotors still howled. When I replaced rotors with SLX rotors all is good.
    The only easy day was yesterday.

  8. #8
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    I had the same issue with new guide rs. Extended the pistons one by one, cleaned with IPA and greased with sram dot grease. Seem ok now.

  9. #9
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  10. #10
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    Can someone explain to me why the caliper piston seals supposedly swell when exposed to brake fluid (why sources say to not try and lube a sticking piston with brake fluid)? Under normal operating conditions aren't the piston seals constantly exposed to brake fluid?

  11. #11
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    I just rebuilt mine. The sram video on YouTube calls for brake fluid as the lubricant for the caliper piston seals when doing a rebuild. The dot grease is only used on the 2 seals that go sandwiched between both calipers. They specifically say not to use dot grease on the piston seals but only dot 4 or 5.1 fluid. I also wondered about this but it may only be in reference to mixing mineral oil and dot fluid with each other.

  12. #12
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    Yes, I noticed that in the SRAM maintenance doc as well. So then where is all this talk about caliper seals swelling when lubricated with brake fluid coming from??? I trust that mechanics who have faced this many times know what they speak of, but then why does SRAM say something different?

  13. #13
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    So, it seems that I may have this same issue. New pads and rotors work great for about 25 miles of single track and then they warble (for lack of a better word).

    Will organic pads help at all, or do you guys think the issue is in the calipers?

    Also, this is only on the rear brake. The front is perfect.

  14. #14
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    Quote Originally Posted by Bike Therapy View Post
    I just rebuilt mine. The sram video on YouTube calls for brake fluid as the lubricant for the caliper piston seals when doing a rebuild. The dot grease is only used on the 2 seals that go sandwiched between both calipers. They specifically say not to use dot grease on the piston seals but only dot 4 or 5.1 fluid. I also wondered about this but it may only be in reference to mixing mineral oil and dot fluid with each other.
    That's incorrect. Any automotive brake system using dot 4 or 5.1 being rebuilt for harsh conditions gets dot compatible grease applied to the pistons before assembly.

    SRAMs RSC rebuild doc even states this. I've rebuilt 7 pairs of guides for my bikes and used Castrol red rubber grease ( dot compatible ) on all pistons, significantly aiding even piston travel


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