Well, I live in new England so it is very wet here. After I cleaned the old caliper up, I was particularly careful to thoroughly bleed all the brakes. Probably more careful than usual. Not sure about that though. I am convinced that moisture contamination is the culprit and I am trying to stick to a complete bleeding of all brakes on all my vehicles on a regular basis. Maybe 60K miles.
In the past I have regularly changed the brake fluid in the master cylinder reservoir. But that clean fluid never was able to work it's way all through the system because when I bled the slave cylinders the fluid came out filthy even though the fluid in the master cylinder was pristine.