XT M765 Disc Brakes - Problem

ew77
ew77 Posts: 57
edited March 2010 in MTB workshop & tech
Having a problem with my XT M765 (dual control) disk brakes. Recently the front brake became next to useless in operation. The lever still has the normal amount of travel. Cleaned up the pads and disc but still no improvement. Replaced pads for new and all back to normal. Following ride the brake has gone back to useless (with the new pads still installed), and again cleaning the pads and discs has not improved it.
Any ideas? Are these brakes prone to leaking fluid, as this is the only thing I can think of that might be causing the problem, but can't see any trace of leaking fluid.
Thanks in advance.

Comments

  • supersonic
    supersonic Posts: 82,708
    It does sound like something is contaminating the pads from somewhere. Is any spray getting on there/ Have you cleaned the bike since?
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  • The Northern Monkey
    The Northern Monkey Posts: 19,136
    old pads coul have been contaminated and new pads not bedded in properly?
    Santa Cruz Butcher
  • ew77
    ew77 Posts: 57
    Thanks for the replies guys.

    Supersonic - I have cleaned the bike since, but I'd assume if it was something I was using it'd more than likely affect both front and rear?

    bigbenj_08 - the new pads worked well straight away and had plenty of bite, but now I can hold them on full whack and just keep pedalling forward!!
  • The Northern Monkey
    The Northern Monkey Posts: 19,136
    ew77
    Sounds like they weren't bedded in properly before you're ride and are now "shiney".

    but if they squeal at all, they are probably contaminated again.
    Santa Cruz Butcher
  • stubs
    stubs Posts: 5,001
    Are the pistons siezed in the caliper I had a problem with my XT brakes same as you lack of power. Basically the pistons were siezed up with road crud, salt and the like.

    Took caliper off took out pads cleaned everything till it was spotless with cotton buds and Iso Propyl Alchohol. Put a couple of drops of Shimano brake fluid (its oil not DOT) on the side of piston let it soak into seals for a while. Then got to work pushing the pistons in with a wooden wedge, pumping them out and working them in and out till they felt smooth. Cleaned everything up again with cotton buds and IPA. Cleaned the disc with fine emery just span the wheel and lightly held the emery between finger and thumb just taking all the ground on crud off not grinding the metal. Reassembled with new pads, bedded them in and they have been fine since.
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  • ew77
    ew77 Posts: 57
    edited March 2010
    I think it may be similar to stubs' suggestion. It appears the pistons are not travelling far enough to apply the brake properly. As a test I taped a washer on the side of one of the pads to get them closer to the disc, and with a set of pads I know are not contaminated (off the rear brake) it seemed to work.

    Would this lack of travel be down to the piston seizing up or needing bleeding (or both?). The lever is not spongy in the slightest and theres a definate point where the lever stops, but I can't seem to pump the front like I can the rears to make the pistons come out further.

    Any further advice would be much appreciated.

    Cheers
  • JamesBrckmn
    JamesBrckmn Posts: 1,360
    bigbenj- what do you mean by bedding in pads properly? is there a way you are supposed to do it? i thought they bedded in by just riding and braking? :?
  • ew77
    ew77 Posts: 57
    Well, took the expensive route and bought myself a new set of XT M775 disc brakes front and rear as a treat for my bike which has sorted my braking problem!

    Will give me a bit of time to play about and try and 'un seize' the piston in the M765 caliper and once done I'll throw them on my Rockhopper HT.

    Thanks for the advice

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