Rear Caliper - Helped Needed - Pictures of Issue - Stuck Piston

[Dave48]

Glad you got "offending" piston out. As you have discovered the grooves for seals must be spotless for seals to fit properly. The dust seals are the first line of defence for the piston seals & are "lipped" to form a close sliding fit against piston. Do yourself a favour and use a little red rubber brake grease on seals -it will make it much easier to fit pistons-failing that use clean brake fluid as "lubricant" for seals..
The small black washer is the seal between the caliper halves-just make sure it sits in recess before tightening bolts :eek