The pistons both compressed in ok, and since my pads were shot, they started out very extended. I don't think my problem here is related to the caliper piston so much, as it is the guide pin.
The guide pin with the rubber sleeve was (and now still is) sticky to begin with, leading to a completely frozen pin later on. I can tell I'm likely on the same path again, because it's still several times stiffer than the other pin, even though I thoroughly cleaned everything up and lightly regreased.
Over time, this goes from sticky to stuck, and seems to be stuck strongly enough to prevent the caliper piston from being able to move it. The pad then wears on an angle.
The only thing I can think of to do, that I haven't already done, is to get a new rubber sleeve for that pin. Perhaps the old one is swollen with age or chemicals. But getting into new aftermarket parts, you'll find they have all sorts of different setups here, and I don't trust that. Ford did this for some reason, and I need to understand what that reason is. It seems like either A) my sleeve is swollen and needs to be replaced with an OEM one, or B) It's tigher by design; possibly to slow down the rate of change that side of the pad moves at.
