Check to see if the parking cable is in place. Not sure if this is a model specific procedure, but on my SE I had to tighten up the brake as the pedal went almost to the floor. This is done by first tightening the brake at the rear wheels, and if that isn't sufficient, you can tighten the cable underneath the car.

Is the ratchet function engaging?

__________________
'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E