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I took the front wheel off my mountain bike and the hydraulic disc pads closed. I’ve opened them back up and replaced the wheel. However, the pads seem to now be closing up all the time and binding on the disc.

Why is this happening, and how can I resolve it?

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Did you change anything with the hydraulic system? Bleed the breaks, check the levels, anything? It sounds like there may be air, or a leak, in the system allowing the pads to re-clamp. – Jack M. Jun 4 '12 at 1:40
What brand and model of brake do you have? It matters a lot in a situation like this. – zenbike Jun 4 '12 at 6:18

You may not have opened up (manually reset) the brake pistons sufficiently. Typically, I remove the brake pads and use a plastic tire lever to push pistons back as far as possible. If do not push hard enough you will not fully reset the pistons and the pads will still rub.

Aside - for those new to hydraulic brakes, most systems automatically set the pad rotor distance to accommodate for wear whenever you activate the brakes. If remove your wheel and squeeze your brake lever, the pistons reset for a closer distance because the rotor is no longer there to stop the pads. As a result, you cannot get your wheel and rotor back in. You fix this by resetting the pistons as described above. Also if you change your brake pads you will also need manually reset the pistons.

You may also find this Shimano tech document on resetting Shimano hydraulic brakes useful. Most other hydraulics will operate in a similar manner.

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