1.Brake fluid is hydroscopic(it sucks moisture out of the air).The brake system
must vent to the outside air as designed,to allow the fluid to move the pads
towards the rotors.
2.Old brake fluid (black or dark brown in color)is acidic(not good).M.B. requires
fluid replacement every two years(Cheap way to assure you stop when the
pedal is depressed)
3.Old brake hoses tend to collapse and not allow the pads to fully retract from
the rotors.
4.Brake dust loves to glom onto everything it comes into contact with...say the
interface between the pads and the caliper(and caliper bolts)...possibly further
compounding the sticking problem.
5.#3and #4 can cause sticking calipre(s).
6.Very worn pads can allow the piston(s) in the caliper to overextend and
stick.

At the very least you need to see if the rotors have enough meat left to
have them "Turned"(Shaved down to a flat surface)otherwise the damage
to the rotors will kill a new set of pads.You'll get to replace the shoes inside
the parking brake drums(you should turn the inside of the drums if any
damage shows)Press the pistons back into the calipers per the manual...
if they are damaged or the seals or bores are damaged rebuild or replace.
Replace the brake fluid...I'd replace the brake hoses too , they are inexpensive.