This is a relatively frequent problem. There are a number of theories in this Forum about it, ranging from the rubber hoses that jump from the steel brake lines to the calipers going soft and somehow sealing the pressure in the caliper, to the master cylinder somehow not releasing the pressure when you take your foot off the pedal.
I personally do not believe the hose theory as the amount of fluid that needs to move when you step on the brakes is very small, it only makes up for a less than a mm travel of the pads, the swelling of the hoses due to the increased pressure and the flexing of the caliper as the disc is squeezed. By design the hydraulic system is not supposed to push fluid around as otherwise the brakes would react at different times and unequally, depending on the length of the flow path, etc.
I suppose it is possible your brake pedal mechanical linkage could be keeping some load on the master cylinder and that could cause the problem.
My Mercedes manual tells me it is the caliper piston seal. Under normal operation the piston seal, a square cross section rubber ring that sits in a recess in the caliper cylinder bore, deforms and stretches when the piston moves to squeeze the disc as you press the brake pedal. When you let your foot off the pedal, the source of pressure is released and the lines shrink back to normal diameter, the caliper "C" shape is restored to its rest position, and the rubber seal relaxes, pulling the caliper piston and pad back away a fraction of a mm from the disc.
When the seal wears, it loses its grip on the piston and then there is nothing to pull the piston and pad back from the disc. Consequently the pad stays in contact with the disc and the disc heats up. As the heat penetrates the caliper the brake fluid can heat up, and swell, and the process of imitating the application of pressure to the brake pedal starts and you get more drag, then more heat and so on.
The fix is to have new calipers installed from a reputable supplier. It is relatively frequent that the rebuilt calipers do not have properly matched pistons and caliper cylinder bores so the rubber seals do not start with the design pre-load and you can have the same problem again. The M-B manual has a test that measures the pull back of the rubber seal, and describes the symptoms of not having the pull-back, then tells you to rebuild or replace the calipers.
Hope this helps. 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)