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mechanical brake caliper not working - possible piston problem?


I'm having trouble with my rear brake, it doesn't have any stopping power, i have changed the pads, ensure the wheel is seated correctly, de greased everything i can get too (which has helped with the smoothness of the piston). adjusted the lever and cable. set the distance for the non moving pad. i have also cleaned the rotor

the rear lever is set to match the front lever eg travel and resistance all feels the same but no luck.

am i missing something or is it new caliper time? i have clarkes CMD-8 mechanical discs,

pic is on the old pads -


  • reaperactualreaperactual Posts: 1,183
    edited October 2020
    This sort of thing usually comes down to set up. Seeing as the caliper has been off and back on again it might be worth re trying setting them up again just to be sure static pad is as close to rotor as possible (almost touching) and aligned so pads are all square against the rotor etc.

    Mostly cable disc brake calipers are pretty bomb proof and last for donkeys years in mechanical terms.

    Maybe replacing cables could work if original ones have been used for a long time, the inners could have frayed inside outer cable housing?

    Have you had the new pads in long and given them a chance to bed in properly?🤔
  • Hi, that's for the reply

    I don't think it is the cable as there is no improvement when the caliper lever is pulled, although we do have a cable to try.

    all the pads are fully adjusted and where they need to be, have been on and off several times today trouble shooting,

    As for running in, been round the block a couple of times but don't really feel safe going too far with as good as no rear brake,

    I've read that lightly scouring with fine sandpaper works? Worth a try?
  • reaperactualreaperactual Posts: 1,183
    Sand paper is more for light pad contamination, if you think it's a possibility then worth a shot Pal. After a sanding the bedding in process is back to square one.

    Seems like you've tried everything else and set up is spot on so I'll try and come up with other causes from memory of cable disc brakes.😆
  • reaperactualreaperactual Posts: 1,183
    edited October 2020
    Hi Bluesocks, how you getting on? A little search around seems the only complaint with the CMD-8 is the return spring is weak. Obviously not the cause for loss of power.

    How much of the overall cable pull arm travel is being used up before the pads contact the rotor and is there enough travel left to increase bite power?

    Just guessing but perhaps the internal cam mechanism has worn down over time which could be the reason for loss of power?🤔

  • update - moved the inside pad to just touching the rotor and did a couple of km no change. couple of vids on how it is set up below (hope it works)

    changing the cable tommorow other then that i think its new caliper time
  • reaperactualreaperactual Posts: 1,183
    edited October 2020
    Excellent clips, very helpful Bluesocks!👍Looking at the second clip I think the problem is the inboard edge of the rotor is touching the caliper body and not contacting with the inboard static pad. Your effectively at 50% braking force.

    Undo the caliper fixing bolts, wind the static pad in more towards the centre of the caliper then re-alighn the whole caliper more towards the spokes positioning the rotor in the centre of the groove.
  • reaperactualreaperactual Posts: 1,183
    edited October 2020
    On another thing I noticed with the set up is the spacers underneath the mounting points which I don't think should be there.

    This is backed up by the first picture of the worn pads, there seems to be a step forming at the top edge which suggests the caliper is too high and pads are not in full contact with the rotor braking band/track. This should add a little more power also.
  • JBAJBA Posts: 2,852
    I agree with reaperactual.
    It looks like you need to remove those spacers to ensure the full pad covers the rotor. Then move the calliper inboard a few millimetres and wind the fixed pad out.
    “Life has been unfaithful
    And it all promised so so much”

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  • thanks guys, ill give this a go tomorrow, massive learning experience for me but enjoying doing it
  • mully79mully79 Posts: 904
    it looks to me like the adapter that the calliper sits on is flipped the wrong way.
  • So spacers removed, cable changed and caliper realigned and it stops!! Still not where it should be but hopefully better once the pads have worn in, thanks for all the help guys 😁
  • reaperactualreaperactual Posts: 1,183
    Glad you got it to work Pal.😎👍
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