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  1. #1
    Axe
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    m785 caliper body leaking (not piston, nor banjo) - is it dead.

    I have just assembled a set of brakes from new XTR trail levers I got - and a pair of XT m785 calipers I have had in a box for well over a year. I do not remember where I got them, I think I could not resist some mad cheap deal ordering online.

    Well, I guess there was a reason for mad cheap - after assembling, and bleeding, firm application of brakes makes them seep oil all along the seal of the two piece caliper body. WTF. Never had that one before.

    Is there is some seal there that can be cleaned, or should I just throw them to trash, as a lesson learned.

  2. #2
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    There's an o-ring between the 2 caliper halves on the banjo bolt end of the caliper. Sometimes the o-ring can dry out a bit if the brake's been sitting around too long without oil in it. Tighten down the banjo bolt as much as you can, then let the brakes sit for a day or so for the oil to penetrate into all the seals. They should stop leaking, or at least leak less the next day.

    If not, then things get fun. Disassemble the caliper and remove the o-ring, give the caliper halves and the o-ring a good cleaning with alcohol to remove the mineral oil. Then, use a small amount of silicone caulk to glue the o-ring into one half of the caliper, then let it dry & cure overnight. Next, put a small amount of caulk on the other half of the caliper around the oil port, taking care not to block the port itself. Reassemble the caliper and tighten the hose & banjo bolt into it to clamp everything firmly in place, let it cure for a day, then fill it back up with mineral oil. That should stop the leak for good.

    This is the silicone caulk I used when I carried out the repair.
    GE Silicone II Gasket & Seal Is Waterproof and Airtight, and Resists UV, Oil, and Grease
    Last edited by aerius; 04-14-2013 at 03:26 PM.

  3. #3
    Axe
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    Thanks.

    It still weeped, after couple days and tightening the banjo as hard as reasonable. I guess I will open one and take a look whats inside. Never had to do any internal maintenance on Shimano calipers and levers, they all just worked.

    Kinda disappointing that they switched to two-piece caliper in XT. That said, other two sets I have are working perfectly fine, so do 596 and old XT on other bikes in my household. I guess it was not a good idea to let it sit for almost two years in the parts bin. #firstworldproblems
    Last edited by Axe; 04-14-2013 at 01:24 PM.

  4. #4
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    Time to break out the silicone caulk! I've run into a few leaky sets of Shimano calipers over the years. One was an M785 last year which looked like it had been sitting in a crappy warehouse for ages judging by the dust and moisture marks on the box. It seeped oil from between the caliper halves for a day or so before the seal swelled up and did its job. Another one was an M755 caliper which had sat around in my basement for 10 years, this one never sealed up so I had to take it apart and caulk it around the o-ring. It's been working fine for over a year so I'm pretty sure the fix works.

  5. #5
    Axe
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    I took them apart, and replaced O-rings with identical o-rings that had been around the bolt that was holding it together instead of the banjo. Those looked nicer - internal o rings seemed to have something resembling a small ridge - maybe for being compressed dry.

    I have smeared tiny bit of that gasket caulk right around the ring - after assembly it seemed to fill the gap. Not sure it matters - under operation that gap seems to be pulling apart ever so slightly.

    Will refill tomorrow and check.

  6. #6
    Axe
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    ...btw, rock solid since then..

  7. #7
    AKA Mr.Habanero
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    Good to hear! I might have the same issue as you did. My front brake kills pads. No matter what l do they squeal to high heaven. Two sets of finned xt pads and a few cheapo Trucker pads. They seem to last longer then the xts. I do the sanding/alcohol thing yadda yadda, same story. I dont want to replace my rotor.
    So. It looks like i will learn how to take apart calipers!!! Yippy(eye roll)

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