Disk brakes retract due to the resilence of the rubber seal for the piston. When this seal gets old or hard from overheating, or gets dirt around it from a torn dust seal, the piston will no longer retract immediately. If it is really stuck, and only goes out, rotor runout won't push it back, either, and the brakes don't completely release.
This usually shows up as jerky stops -- even if you reduce pedal pressure as the car stops, the pressure on the pads doesn't go down fast enough and the car jerks as it stops. Can make the brakes groan or squeal, too, depending upon what is stuck where -- Volvos are really bad at this because they have two pistons side by side in the caliper.
The fix it to replace the piston seal, or the caliper, depending upon condition. All caliper will eventually go bad as the seal won't last forever, but this is on the order of 30 years if the brake fluid is changed. Most cases are from dirty brake fluid (never changed) and attendant corrosion, or, more typcials of Benz, running rotors too thin so the pistons cock in the bores and stick, overheating the calipers, ruining the seals and dust boots.
Parking brake shoes can get stuck, too, or be adjusted too tight, so they don't release.
Dragging brakes will cause poor fuel milage, overheating, and odd braking on slippery surfaces, too.
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!