Brake piston fell out during bleeding - anything to fix?

Question

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?

Answer 1

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.