Two things are very bad for MB brakes -- running the rotors down too thin, sticking the calipers, and overheating them and letting them sit for extended periods.
I think extended sitting leads to the collection of water under the dust shields, and hence corrosion of the area of the caliper bore above the piston seal. This area does not appear to be chrome plated, for some reason, and any roughness here causes the piston to stick.
I would remove the caliper and replace the piston seal, dust boot, and heat shield. It will seem to work OK when prying it back, usually, but still stick once hot in use.
And rusty pads will certainly cause trouble -- I was getting concerned that I had 60,000 miles on the front pads in my Volvo with no signs of worn out pads, although I had some squealing issues (not uncommon on the 740). When replaced them, I discovered that they had been rusted in place for some time -- took a large hammer to drive them out AFTER lifting the floating caliper! I greased the new ones after scraping off large amounts of rust. The old pads were worn tappered from being more stuck on one end than the other, causing the screech. They didn't work well, either -- I was stopping only with the rear brakes!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!