Semi-stuck piston makes a lot of sense...
As a matter of form, I always scrub out all of the old brake dust and paste that tends to gunk-up these calipers--though I've always used canned brake parts cleaner vs. brake fluid to do the job. (I also use a piston-spreading tool to evenly compress the caliper pistons, which greatly eases insertion and removal of pads.)
Hopefully, between the manual spreading of the pistons and the thorough cleaning I gave the caliper, any stickiness that may have indeed existed has now been corrected.
I'll recheck the caliper/pads next time I have the car up on stands.
Thanks for the good suggestion!
