It's a bit of a feel thing, but when moving the piston in with a lever, one backs off the force and the piston should be observed to move out a small amount due to the distortion of the seal.

You've put in a few words a concept I think needs to be taken into account when rebuilding (or as most of us do, replacing) calipers: "distortion of the seal."

This little bit of spring-back, combined with new rubberized shims, if factory type, multiplies backward to the pedal feel by the hydraulic advantage to result in uncounted numbers of posts lamenting "spongy brakes" after the job.

After sorting out just "how spongy" and eliminating the mismatched caliper rebuilds, ruined masters, and bleeding mistakes, there are still some dissatisfied with the lack of the hard pedal their stiff-sealed 30-year-old caliper pots provided. There's no substitute for experience.


--
Art Benstein near Baltimore

If flying is so safe, why do they call the airport the terminal?