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  1. #1
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    Shimano M775 power loss

    Hi all. My Shimano M775 Disc brakes have suddenly lost significant power. By significant power loss I mean that on a on a moderate descent where before I was comfortable using one finger I now need two fingers and a lot of effort for the same stopping ability. Or another example, when stopped on the flat I could lightly touch the front brake lever with one finger and it would lock the wheel enough that I couldn't push the bike forward. Now with two fingers and a decent squeeze I can still push the bike forward.

    I've been checking out what I should do both here and else where on the net and am planning to start working through things step by step until they're good again. My pads have plenty of wear left. I will replace and bleed the brake fluid.

    Before I start that however I have a few questions:

    Is sanding pads and or rotors an accepted/worthwhile practice? What grit should be used? Any other tips?

    Is rotor glazing a common problem. Something I read suggested if rotors were bedded in properly at the beginning (mine were) it should never be a problem. Can glazing occur mid life?

    Thanks and any help or links to help appreciated.

  2. #2
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    1) probably not going to help this, but go nuts. Finishing sandpaper , lightly pinched around the rotor, spin the wheel.

    2) glazing happens when the rotors heat high enough to melt the resins in the pad, and they deposit on the rotor. This will make the rotor discolored. Brake cleaner will fix it if this is an issue.

    It sounds like your brakes need to be bled, I would start there. If the loss of power was sudden (we're talking mid ride, not rode it last week and it was fine) I would be concerned that a seal broke somewhere in the system. At any rate, bleed it and see what happens.

  3. #3
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    If your lever throw has not changed then bleeding probably isn't going to fix anything. If air is in the line your lever will be 'spongy'.

    I'd start by cleaning the rotor with whatever will break down grease...brake parts cleaner, simple green and finish with IPA (90% or better is nice). Sand them in a circular pattern if you wish...finish with IPA. Then replace the pads. Go bed them in by performing 50 or so quick sprints and hard stops....or you could just ride. See if problem goes away.

    You can bleed the brakes 1st if you want but if it were me I'd start with the easier stuff.

  4. #4
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    I've done what I can with the brakes and they haven't improved, in fact if anything, they're even weaker.

    Please note my rear brake seems perfect. There's a nice strong grabby feel at slow speeds on pavement. The front brake is woefully inadequate.

    What I have done so far is:

    Completely flush out the old brake fluid until clean was coming through. While doing that I didn't find any bubbles or anything else suspicious looking. Test rode and no change.

    Took out the pads and thoroughly cleaned the pads and rotor with disc brake cleaner and cleaned the rest of the caliper (although it really wasn't very dirty). Test road and no change.

    Finally, replaced the metal pads with my "old" used, broken in and ridden for a while organic pads which have almost full wear left and absolutely no difference.

    The lever feel bis the same between brakes. The brake overall are in very good condition. My bike is stored indoors always. There are no fluid leaks.

    Some back ground info on the detection of lack of power:

    I get to ride rather infrequently. Over the new year holidays I got out maybe 4 times, each time 3 - 4 hours. At the beginning of that time I hadn't noticed any weakness in braking power. On my last ride I rode some very steep narrow single track and noticed I was using considerable hand power and was struggling to control my speed. I'm not sure if the brakes were slowly losing performance over that time or just during the last ride
    Since the first ride and not noticing any lack of power and the last ride I had not cleaned the bike (meaning no cleaning solutions could have contaminated the bike), the room where my bike is stored has nothing that could contaminate the brakes and I only ride in the dry so the brakes hadn't been through any serious mud or water (perhaps some light water draining across the road but not enough to even get the bike wet).

    In regards to the cleaning I tried to buy some IPA. I live in Japan and couldn't find any locally. I COULD order it if I really need it. I have automotive brake cleaner that I used. I find it very difficult to translate exactly what's in this type of product and their exact purposed. So, what I'm saying is that I don't really know how much difference there is in IPA and brake cleaner in what they will and won't remove. I would have thought that I'd have had some improvement at least from the brake cleaner.

    So my questions are - is IPA essential in rotor and pad cleaning. Is sanding the pads and rotors my last option?

    Again, any help or advice appreciated!

  5. #5
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    IPA is not essential, anything that removes any grease or residue and leaves a clean finish will work.

    Maybe try this. Swap the rotors front to back and see if anything improves.

  6. #6
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    I have and 180mm front and 160mm rear unfortunately. I'm not sure if contaminated pads and rotors have a particular look or visual clue but everything looks so clean. I'm really not sure what's going on with the brake and why it only seems to be the front brake.

    It my be a week or two until I have a chance to look at it again but when I can I might try sanding and clean them again and also bleed them a again to double check I don't have any air in there and see if there's any improvement.

  7. #7
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    Quote Originally Posted by komekomegaijin View Post
    I have and 180mm front and 160mm rear unfortunately. .
    Just remove the adapter on the front caliper and mount the 160 rotor if you want to check it out.

    Personally I'd just get new pads and sand the crap out of the rotor with fine grit paper. Then give time for the new pads to bed.

  8. #8
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    Degrease rotor, bake pads at 350F for 1 hour. Never failed for me. If you sand the pads, you will have to rebed them in and will probably still be contaminated as the pad material is porous.
    lean forward

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