late to comment but a quick list;
BEFORE you remove caliper, crack loose, retighten, banjo bolt and 12mm head slide bolt.
BEFORE you remove brake line. with caliper off, pump piston out a ways with brake lever.
remove pad spring clip.
remove stainless ring and dust seal.
ad a bit of brake fluid around piston. (q-tip works well)
use a clamp and push piston IN a bit, back off clamp "some" use rubber tipped air gun in line port. to push out piston if it's not sliding easy now; repeat pushing in clamp add more brake fluid push out with air.
yes stuff arag in there and keep fingers out.