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I have pushed the hydraulic pistons fully out, but after a single firm squeeze of the brake lever, they ride slightly too tightly, such that rear wheel doesn't spin freely. It rubs a little bit. How can I permanently adjust the pistons to a wider default set point whenever the lever isn't being squeezed?

Bike is 4 months old, so brakes, rotors, and fluid should in theory be new enough, and visually look good.

When I finely align the brake, then manually push the pistons out, the wheel spins very freely. It's only after the first firm squeeze that it goes back to lightly rubbing.

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  • Which brake? Some models are allegedly more susceptible to that than others.
    – gschenk
    Jan 13 '20 at 21:04
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There is no adjustment for piston position on hydraulic brakes.

Hydraulic brakes have a built in mechanism to retract the pistons slightly when the brake lever is released. The pistons are mounted in flexible rubber seals that deform as the pistons are pushed forward. The seals then pull the pistons back as fluid pressure is released. The pistons also slip through the seals to automatically advance as the pads wear.

What you need to do is adjust the calipers with the pads advanced up to the rotor. If you push the the pistons back and adjust the caliper position you wont be able to align the pads accurately enough.

A good trick is to loosen the caliper bolts, then hand tighten them so the caliper will still move. Pull the brake lever so the pads grip the rotor, keep it pulled while you tighten the bolts enough so the caliper wont move, release the lever and tighten bolts to proper torque.

Park Tool video on how hydro brakes work:

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  • Works like a charm.
    – That Idiot
    Jan 13 '20 at 14:16
  • maintenance - How do self-adjusting hydraulic disc brakes work?
    – Vladimir F
    Jan 13 '20 at 15:02
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    "What you need to do is adjust the calipers with the pads advanced up to the rotor. If you push the the pistons back and adjust the caliper position you wont be able to align the pads accurately enough. A good trick is to loosen the caliper bolts, then hand tighten them so the caliper will still move. Pull the brake lever so the pads grip the rotor, keep it pulled while you tighten the bolts enough so the caliper wont move, release the lever and tighten bolts to proper torque." Pretty sure this just means you finely align the brakes with handle squeezed?
    – Tom Mercer
    Jan 13 '20 at 19:02
  • I edited my question to make it clearer. I have no trouble adjusting the brakes perfectly. Then, I manually push the pistons out. In this state, it doesn't rub. After a good squeeze, it's back to rubbing, despite being perfectly aligned. I imagine I need a way to make the pistons return harder, and rest further apart.
    – Tom Mercer
    Jan 13 '20 at 19:06
  • @TomMercer There is no way to make the pistons rest further apart (at least on road bike brakes I've seen). If you push the pistons back of course you'll have no rub (why are you doing this anyway?). Did you have rub after adjusting the caliper but before pushing the piston back?
    – Argenti Apparatus
    Jan 13 '20 at 19:56

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