During a pad change, if you press the brake pedal with the pads out, or the piston extends too far, it can pop out past its seal - a frustrating mid-job problem. Here is what to do about a caliper piston pushed out too far.
Why it happens
Pressing the brake pedal with the caliper off or the pads removed pushes the piston out with nothing to stop it; once it passes the seal, it can leak or come out entirely; see new pad won't fit. Never pump the pedal with the pads out.
Retract it correctly
If the piston is just over-extended but still seated, retract it the right way - straight-in pistons push back, twist-in rear pistons must wind back; see piston windback. Crack the bleeder while pushing so fluid escapes to a bottle.
If it popped past the seal
If the piston came out or past the seal, air and dirt are now in the caliper, and the seal may be damaged. The caliper likely needs a rebuild or replacement, then a full bleed; see caliper condition. A loaded caliper is the quick fix.
Prevent it
Keep a pad or spacer in the caliper, and never press the pedal while it is open. Retract pistons with the proper tool and an open bleeder. Fit quality pads and hardware.