First pull the rear discs and inspect the shoes to make sure they are still their.
If they are then I am 90% sure you can adjust them like a regular drum brake, their is a click thing you turn with the rotor on. Their is also a spliter up underneath the car in front of the rear flex disc and above the driveshaft. Thats where you adjust the cable, like if the pedal isn't coming up enough to turn the light off. I *think* MB specs 3-4 clicks before it is tight. At least thats where mine is on the SDL, with new shoes 3-4 clicks will hold the car no problem. 90% of the W126's I come across have poorly adjusted or non working parking brakes...
Also according to the manual you are supposed to engage it while driving at 30mph for a few seconds. Never understood why, havn't done it yet I don't want to glaze the shoes.
If the parking brake is working properly you shouldn't need to clamp hard.