rear caliper questions

in the process of redoing the rear calipers. the problem was the left rear outboard piston was hung up. after removing the piston, which was quite difficult, needed to drive it free with a wooden drift, there didn't really seem to be any evidence of the cause for it to be malfunctioning. i wonder if the internal spring could be bad. there doesn't seem to be anything out of line. it certainly has a great deal of tension. i assume the way it functions is the fluid pressure forces the piston out and the spring retracts the piston. correct? if so are there any specs on the force needed to work the retracting spring. next, the circlip on the adjusting screw. where does it seat? the screw has a groove that falls inside the casting but i'm assuming that is where the o ring seal seats so where does the circlip go? at the end of the threads? i've searched the archives to no avail.