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When bleeding a brake, I did not insert the correct spacer, pushed the lever too eagerly, and the piston fell out (and the oil was on the floor).

Is there anything to fix after this besides just putting the piston back and doing the regular bleed?

Bonus question: how is the oil kept inside the "caliper+piston" combo, how is it possible that under normal operation the oil does not leak at all through the caliper-piston interface? I thought there's some kind of membrane connecting the piston to the caliper making the system closed at the caliper end, but there's none?

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    I imagine that knowing the exact model of the brake will help folks answer this question.
    – Paul H
    Aug 25 at 15:14
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    The oil is kept inside through seals. Sometimes they're made of rubber, other times, they're made of metal (piston rings in a car engine keep the combustion gasses in the cylinder and they're made of metal to withstand the heat) and might not be so obvious. It's entirely possible that you've ruined the seal and will have to replace it. Even if it's not obviously damaged, now might be a good time to replace it (them - could be more than one) anyway, since it's disassembled.
    – FreeMan
    Aug 25 at 15:25

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Getting a piston back in doesn't have too many hidden steps. You want to get everything clean with appropriate means - typically this would be a detergent wash that's safe for disc brakes and/or alcohol. You want everything reasonably clean before you reassemble. Then lubricate the caliper bore with brake fluid and just push it in.

The bleed afterward doesn't necessarily have any special steps, but since the caliper needs filling from scratch it can take some extra fiddling to get the caliper air-free. I've had to do it twice before.

Brakes like you were imagining do exist - look up the Ashima PCB ("pancake brake"). They've failed to catch on much and it seems like getting much functional upside out of the design has been difficult.

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