I have seen this problem reported a number of times in this and the Diesel forum, so you are not alone. I have tried to explain what is supposed to happen when the pressure on the back of the pistons in the brake calipers is taken away by the operator moving his foot off the brake pedal. Unless the master cylinder or brake pedal springs have failed so that the residual pressure on the back of the piston is maintained significantly hgher than the static head of the elevation difference between the brake fluid reservoir and the caliper, this is a caliper problem. You can check the residual pressure, or the brake pedal return motion, to see if there is a problem with those features. Each of those options would affect alll the brakes equally. If one is hanging up it is definitely the caliper.
Not all rebuilt parts are necessarily correct. I have had an alternator, sold as a rebuilt unit from the dealer, act like it was working at idle but fail to charge the battery. Of course I assumed the alternator was not the problem, and went nuts trying to find shorts and the like.
Anyway, the piston seal ring in the caliper is the ONLY thing that pushes the pad back away from the disc when you release the pedal pressure, assuming the pedal returns all the way to its rest position and the springs in the master cylinder push the pistons in there back to their rest positions. The failure of one of the master cylinder springs would be unusual in my experience. A mechanical hangup on the linkage from the pedal seems more likely, which could leave the master cylinder kind of preloaded a little.
Sorry I can't be of much more help. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)