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Thread: M785 trubbs

  1. #1
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    Bonking ... not feelin' well M785 trubbs

    I've just built up a new bike with M785s, and am still in the "fiddling about" stage. I'm running 160's F&R, with Icetech up front and a KCNC razor rear. Same problem on both.

    I'm having a real devil of a time with pad/rotor spacing. It seems that I can get them not to rub, but good god! It took me a good 45 minutes of fiddling more apt to a watch repairman than a bicycle repairman. I'm guessing the pad/rotor spacing is no more than 1/100th of an inch, and maybe a good bit less than that. It's effectively an invisible gap even holding a flashlight behind it. After some really, really gentle pushing on the rotors here and there, I'm able to get it not to rub.

    But then! If I remove the wheel and reinstall, we're back to square one . There's just no way that a removal & reinstall of the wheel has a sub 1/100th inch accuracy. I've done this three times, and each time it's taken me another 45 minutes of swearing to get the things not to rub.

    I've googled around and it seems these brakes have no way to adjust the pad/rotor spacing.

    So: I don't get it! A field repair of a flat seems right out of the question, unless I want to hump the bike home or ride with rubbing brakes the whole way back. Yet I know these are popular brakes, and get good reviews. I learned to ride back in the 60's when pad/rim tolerances were measured in entire millimeters . Feeling out of my element here.

    I have no prior experience with hydraulic disk brakes. Surely they can't be this fiddly? I can get them not to rub, but with such a minuscule, sub-1/100th inch tolerance, it seems all but unusable outside my garage. I'm afraid to go hit the trails.

    Help?

  2. #2
    Drew
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    M785 trubbs

    loosen the post mount screws. hold the brake lever in and retighten the screws. release the brake lever. if that doesn't fix the problem you likely need to true up your rotors.

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    yeh try that (but it never works). You will get quicker over time and sometimes you luck out and get it on your first try. Someone mentioned sliding business cards between pads n discs and then adjusting them, never tried it yet.

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    Thanks guys. I did try that and it might have helped position the brake on the mounts, but fundamentally it didn't change the spacing between the pad and the rotor, which I think is the root of my problem. My rotors are pretty true, within that 1/100th inch tolerance anyway. When they rub, they rub pretty evenly all the way round. I wonder why they call for such narrow spacing. It seems like they could spec half a millimeter, or even a full mm, and everything would basically be fine. Plus, it'd be way easier to remount the wheel. Certainly you don't want too much distance, because it would start to eat into your mechanical advantage if the pistons have to travel too far. But a mm seems... OK .

  5. #5
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    Re: M785 trubbs

    Push the pistons back

    Sent from my GT-N7105 using Tapatalk 2

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    Quote Originally Posted by petercarm View Post
    Push the pistons back
    That doesn't change the pad/rotor spacing once you squeeze the levers again in normal riding, does it? The problem isn't getting the wheel in place, the problem is getting it in place within the same 100th of an inch in 3 dimensions that you need to avoid it rubbing on the rotors from being positioned ever so slightly differently. The through-axle just doesn't position the wheel with that degree of accuracy. It's good... but not that good.

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    Re: M785 trubbs

    Yes it does. The spacing is set by the seals in the caliper. If you are not starting from the pistons pushed back the spacing ends up to close.

    Push the pistons back. Align the caliper by eye with no pads in. Pop in the pads. Job done.

    Sent from my GT-N7105 using Tapatalk 2

  8. #8
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    Those brakes have servo-wave, this means that there's something wrong with yours as the pad-rotor clearance on my M596's and my mate's M785's are more like 1-2mm on each side.

    Did you get them pre-bled?

    One solution would be to do a bleed using the right piston-spacer block from Shimano.

    Remove the wheel
    Remove the pads
    Push pistons in all the way
    Insert bleed-block into caliper
    Attach hose to bleed nipple on caliper
    Remove bleed port screw on lever and thread in a plastic 5cc syringe
    Add some oil to the syringe, about 2,5cc
    Open bleed nipple on caliper
    Pump lever 3-4 times
    Hold lever in and close the bleed nipple, release lever.
    Remove tubing and syringe
    Replace bleed port screw
    Clean up with isopropyl alcohol
    Remove bleed block, replace pads
    Re-install wheel
    Pump the brakes 4-5 times
    Re-align caliper.

    This SHOULD fix the issue.

  9. #9
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    Quote Originally Posted by divernick View Post
    Did you get them pre-bled?
    Yep, pre-bled. I will try the respacing you suggest with the spacer block. I think you and Petercarm are saying something kinda similar, so hopefully this will do the trick. Thanks! I will try, and report back.

  10. #10
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    Zarniwoop,

    Just for eliminations sake, push the pistons back in (carefully and gently with a plastic tire lever) as Petercarm said. You should have more clearance than what you're saying but regardless of that, taking the wheel out and putting it back in shouldn't give you completely different results. This points to something effecting the way the wheel is sitting in the fork, ie. the axle may have a 'slight' warp to it.


    Try this. Push the pistons back. Adjust your calipers so you have clearance on both sides (even clearance, and no catchy sounds when your pads first come in contact with the rotor). Then, take a sharpie marker and put a small mark on the axle and the forks dropout, corresponding with each other. Take the wheel out, put it back in with the axle 'clocked' in the same position again. (line up the marks) If it is running fine, take the wheel back out and put it back in with the axle randomly positioned. If it rubs, it's your axle. Return the axle back to the marks to double check that is indeed still working fine.

    From what you describe, this seems like a place to look. I have two XT front hubs that were doing this on two separate bikes.

  11. #11
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    It may be the hub too if the rotor is centered to begin with but wheel removal and reinstall results in a rotor that is not centered any more. I found that some hubs lock nuts or end caps were not absolutely square. Has a pair of XTR hubs where I marked the lock nut and reinstalled in the same orientation and that was the only way to not have to recenter the calipers again.

    And I agree with the others too. Shove those pistons all the way in. The square seal flexes as the pistons go out when you pull on your brake. The seal flexes back to return the piston in its bore. As the pad wears the piston slides against the seal which is how they retain the spacing as they wear. So when you push them back in you can get the same process in reverse. It might help to remove the pads and pull the lever to expose more piston, lube that with brake fluid and push them all the way in. That may help lubricate the seal so that it lets go and lets the piston go back into its bore. And you will need to push the piston back in numerous times until that seal lets it go in all the way. Then you should have greater clearance.

  12. #12
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    And by brake fluid, he means Shimano Mineral Oil.

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    Quote Originally Posted by jochribs View Post
    taking the wheel out and putting it back in shouldn't give you completely different results.
    Well, it's not much different. But when we're talking a hundredth of an inch, it doesn't have to be much different to matter. I'll try your rotational symmetry experiment though; it's certainly an interesting idea, just in case it does help.

    I haven't gotten around to bleeding the brakes, but I did try removing the pads and pushing the pistons back with the yellow spacer block from Shimano. That didn't seem to help. After reinstalling and applying the brakes, I'm back where I started: no clearance to speak of. I'd love to see maybe half a mm on either side, if I only knew how to get it.

    When I get more time over the w/e, I'm going to try the bleed. Man, LBS's really nail you on the price of that Shimano brake oil!

  14. #14
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    Could also be that there's too much oil currently in the system, like as if the pistons weren't all the way in the calliper when they were bled....

  15. #15
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    Quote Originally Posted by Zarniwoop View Post
    Well, it's not much different. But when we're talking a hundredth of an inch, it doesn't have to be much different to matter. I'll try your rotational symmetry experiment though; it's certainly an interesting idea, just in case it does help.

    I haven't gotten around to bleeding the brakes, but I did try removing the pads and pushing the pistons back with the yellow spacer block from Shimano. That didn't seem to help. After reinstalling and applying the brakes, I'm back where I started: no clearance to speak of. I'd love to see maybe half a mm on either side, if I only knew how to get it.

    When I get more time over the w/e, I'm going to try the bleed. Man, LBS's really nail you on the price of that Shimano brake oil!
    I know what you mean about not being much different, but it shouldn't be different at all. I am probably not helping much with my axle recommendation because I was referring to quick release. I don't have much experience with thru axles, unfortunately. I just noticed that you mentioned having one early in the thread. I do however recall a thread some time ago (last summer?) about issues of the wheel not squaring up, or something like that....when you tighten the axle... something of that sort. I just can't remember exactly what the issue was.

    It might help to do a search about the thru axles. I think your problem is emanating from the axle/dropout area. I'd do a search but gotta run to work. I'll look into it when get home.

    Are you able to put the bleed block in without trouble or having to push the pistons in? Divernick might have a point with too much oil. If it is tight getting that block in, once you have it in, put a bleed tube with a little oil in it on the bleed valve, and open the valve. A little more oil should come out and into the tube if this is the case. Then tighten up the bleed valve and all should be good.

    Just throwing this out there as well...I saw that this is your first pair of hydraulics. Do you know not to squeeze the lever when there is nothing in the caliper? (pads/rotor or bleed block)

    Also, get the big jug of Shimano mineral oil...Cheaper that way

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