The parking brake doesn't make sense and you should be able to tell by pulling on it by hand (from underneath the car) as to if it works properly. http://vnc.thewpp.ca/stuff/bentley/e...2/ch7.2.1.html the fact that you disconnected it and it still drags confirms this unless there is a mechanical failure after the cable but that still doesn't make sense. Since you changed the caliper perhaps you have done something wrong? A second set of eyes and a manual wouldn't hurt.

If they go on fine and then drag something is not right. Who did them previously? You say they are "six months and in perfect condition" but they obviously are not perhaps there is a relation with the current problem? Did you examine the old caliper and were there any defects? If there is nothing wrong with it then you need to look elsewhere.