Try resetting vacuum pump which is located in trunk. This is only a temporary remedy.I believe it is the #9 fuse on my 1992 500sel.Remove and re-install this fuse and see if it works.Your vacuum pump or a leak in a line is probable culprit. Their is a lot of info here on door assist problems. Good luck.P.S. I reset mine 3 months ago,everything is still fine. Also check hose connection at rear trunk lock for a leak.