When I first bought my W140, I had the same problems occuring...ie the doors would self seal but the trunk handle would not always extend or retract and the trunk would never close on its own. I obviously never figured it out except, 6 months after purchasing the vehicle, the pump went south. Dissassembly showed one motor brush severed in half and the other with just a hair of brush remaining.

Once the vaccum pump was repaired, the rear units started working properly (even the backup guide arials until my son tried to help and push the arial in subsequently rupturing one of the internal diaphrams.)

So, in all probability, the vaccum/pressure pump motor in yours is weak and not supplying sufficient volumes of vaccum/pressure. Unfortunately its an expensive replacement.

Best of luck though. If you know someone with a working unit (friend, partner etc), try substituting their pump and see if everything works.

Ron

__________________
1995 E320 -171k-km
1995 S420 -333k-km