Rear m615 brake issue

mathematics
mathematics Posts: 453
edited October 2017 in MTB workshop & tech
I've posted about spongyish brakes before. Since then I've bled them numerous times and changed pads etc.

But, although not hugely spongy there is a lot of travel in the lever before bite - almost to the handlebar - with the little bit of spongyness I sometimes trap my fingers between lever and bar. So whats the issue?

Ive looked for leaks but can't see any, bled them life out of them using Shimano fluid and the Epic method.

I've just taken the pads out and the callipers off and operated the brakes alone - the pistons do not move the same distance, could this be it? I've just tried lubing the piston with brake fluid but this doesn't seem to have helped yet.

Any help would be appreciated and I have a trail centre ride coming up at the weekend.

Thanks

Chris

Comments

  • The Rookie
    The Rookie Posts: 27,812
    It's vanishingly rare for the Pistons to move the same amount when squeezing the lever with no resistance, as some basic thought would tell you.

    Have you set the freetsroke adjust?
    Have you looked at the pads to see if you have excess movement before they grip the disc?
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • mathematics
    mathematics Posts: 453
    The Rookie wrote:
    It's vanishingly rare for the Pistons to move the same amount when squeezing the lever with no resistance, as some basic thought would tell you.

    Have you set the freetsroke adjust?
    Have you looked at the pads to see if you have excess movement before they grip the disc?

    Thanks for your reply.

    The free stroke adjust? Not sure it has this, but if I adjust the levers right out i'm stretching to reach them.

    The pads do move a distance to reach the rotor but whether this is excessive i'm unsure.

    Why would the pistons not move equally? I would assume the fluid is pushed into the caliber and distributed evenly side to side??
  • The Rookie
    The Rookie Posts: 27,812
    mathematics wrote:
    The free stroke adjust? Not sure it has this, but if I adjust the levers right out i'm stretching to reach them.

    The pads do move a distance to reach the rotor but whether this is excessive i'm unsure.
    Sorry SLX doesn't have free stroke adjust, but it does sound like the pads are sitting to far off the disk, you'll never sort that with bleeding (in fact it makes it worse as you lose however much they brakes have self adjusted when you do the bleed.
    3 options may help
    1/ Clean up the pistons by advancing them out, cleaning round them, pushing them back, but it doesn't sound like that is the issue
    2/ 'Advance the pistons' (commonly needed on avids) by pulling on the brakes with no disc in between so they adjust out closer to the disc
    3/ Rubber band method where you rubber band the lever back to the bars overnight to try and get them to adjust closer to the disc.


    mathematics wrote:

    Why would the pistons not move equally? I would assume the fluid is pushed into the calliper and distributed evenly side to side??

    The pressure is distributed equally, the only way volume can be is with two pistons at the lever and two hoses, one piston will always have a lower friction to move than the other and it's not uncommon for only one to move at all when nothing is constraining the easier moving piston to increase the pressure enough to overcome the friction on the other one.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • mathematics
    mathematics Posts: 453
    Well I cleaned up the pistons and this seems to have worked for now - only time will tell if the problem is fixed - I amy need to look after my brakes little more!!
  • dcwhite1984
    dcwhite1984 Posts: 86
    Don't forget to align the calipers and pads when sorting them, there's a decent GMBN video on YouTube showing how to do this.
    If they are misaligned they can cause brake rub or other symptoms which you are suffering.
  • mathematics
    mathematics Posts: 453
    So the saga continues.

    The brakes have worked well after cleaning and adjustment - thank you.

    But...

    after last week's and today's ride i noticed greasy 'fluid' in and around the lever almost as if it leaking brake fluid, but I don't think its from the hose connection or the top of the reservoir. Are these brakes serviceable? can I take it apart and replace any seals should I need to?

    I'm going to re-do the hose connection as soon as I can get a barb and olive.

    Can I work on the rest of the lever?

Categories