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Sticky Shimano XT piston in brake caliper

NorthwindNorthwind Posts: 14,675
edited December 2009 in MTB workshop & tech
Just wondering how best to proceed here... One of the pistons in my rear XT caliper is a little sticky. It moves and retracts OK, it just doesn't do so til after the other one has contacted the disc, and even then the disc gets pushed over which seems to be the cause of my bike's banshee wail and uneven pad wear. I've centred the pistons but that makes no difference. This occurred before, so I stripped and cleaned the piston, cleaned out a little muck from under the seal (though not very much at all), cleaned the piston and seal, and refitted- no change at all.

So. If it were a motorbike, I'd replace the seal, but it doesn't seem like I can buy the seal seperately. So before I get another caliper, is there anything else I should do? After 2 strips I'm as confident as I can be that there's nothing left to clean out of the seal's groove, and the piston and seal were both clean. I used a little mineral oil to lube the piston the first time, and when that didn't work a tiny amount of red rubber grease the second time, but neither seemed to make any difference. Can a shimano service centre get the seals seperate?

The piston (and lever, in case it's relevant) is an M775. It's definately not just a centering or alignment issue- I can hold the looser piston down and allow the stiff one to move, but it won't do so unless the other one's immobilised.

(A shimano caliper is only £40 and comes with a set of pads so it's not the end of the world, I'd just like to fix it without that if I can!)
Uncompromising extremist

Posts

  • nicklousenicklouse Posts: 50,675 Lives Here
    was the rubber grease DOT or Mineral compatible?

    keep working it in and out.

    you could pop it out and check the seal.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • NorthwindNorthwind Posts: 14,675
    Ah, obviously wasn't clear enough, I've had the piston out, lifted the seal, checked it and the piston for obvious damage, cleaned and lubed then refitted it. Er, 3 times now ;) Once about 3 months ago, which worked a charm for a while, but now the problem's back. Done it twice in the last week, just on the offchance I missed something. It's the outer piston, though I doubt that makes any difference?

    The rubber grease is both dot and mineral compatible, though I did only use it on the most recent full strip and clean so it definately isn't part of the problem. Both of the previous times I used only mineral oil.
    Uncompromising extremist
  • nicklousenicklouse Posts: 50,675 Lives Here
    sounds like something is not in tolerance.

    Last option is swap the pistons over as it could be the other one that is wrong and too slack.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • NorthwindNorthwind Posts: 14,675
    Ah, good shout, I'll try that, see if the problem travels. Thanks a lot!
    Uncompromising extremist
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