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My dual piston Shimano XT brakes are acting strange... One piston won't return to the right place, so one brake pad can be rubbing all the time, which doesn't let my wheel spin freely...

I have done multiple things to try to fix it. I have used grease, brake fluid, and I have also "exercised" the possible lazy seal/piston. Currently the wheel spins fine from my recent "exercising session", but the piston is still more extended than the other one.

What else could I try? Would it be worth it to buy a replacement caliper cheap from Jenson USA?

  • Hi, two questions. Have you tried to reseat the caliper once you have pushed the piston in? Do yoy have the correct amount of fluid in the system. – dmb Feb 20 at 11:34
  • @dmb I am going to bleed them this week. So I will report back my results. I have tried to center the caliper, but I later figured out that the problem was with the pistons and not the caliper's alignment – JC-aviation Feb 20 at 14:06
  • What I usually do to align the caliper is to push al the way back both pistons. Then mount it with the bolts loose. Then pump the brakes, till the lever is firm and locks the wheel. If there is no one to help you, get a ziptie and tie that lever so the wheel is lock. Then Go and hand tight the bolts. This is crucial for me, tight the bolts just a little bit and do the same with the other, till you achieve the desired torque. This way the caliper won't sway away and it should sit in it's natural position. – dmb Feb 20 at 14:56
  • @dmb thanks for advice on adjusting the caliper. I will bleed the front brakes and let you know what happens – JC-aviation Feb 20 at 15:03
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    Grease is a seriously bad idea as it may get on the rotor. – Argenti Apparatus Feb 26 at 13:37
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Every Shimano hydraulic brake uses mineral oil. The material used for the seals is known to be incompatible with DOT brake fluid so I suspect using conventional grease may have caused the seals to swell irreversibly.

You should never need to grease the pistons of any bicycle hydraulic brake, since the fluid is a lubricant and the pressure seals so tight that they would wipe grease off the piston as it retracts. Grease may also become less viscous when heated, allowing it to drip onto your rotor with catastrophic effects, or may catch fire.

In future, you can use spacers and clamps to restrict the free piston so all the pressure goes to the sticky piston. Ensure the piston doesn't move so far that it leaves the seal. Clean with brake fluid and a soft cloth or q-tip. Push back in and retest. If cleaning isn't enough, the piston's seal surface, the seal or the caliper may be damaged, or one fluid passage inside the caliper could be blocked. Such issues are rarely user-serviceable.

Shimano uses easily-damaged ceramic pistons in its XTR brakes, I'm not sure whether any XT models also have them. Ceramic pistons are definitely Service Center only.

  • If the system uses mineral oil then using conventional grease is probably not the problem (though it's not a good thing to get near the pads or rotors). Rather, using non-petroleum "brake fluid" (if that's what he meant) is likely to have created a problem. – Daniel R Hicks Mar 28 at 12:06

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