Problems with the pneumatic system can cause closing assist and door lock issues.
Car Problem Reports
Mercedes-Benz 300SD Closing Assist/Door Lock Issues Due to Pneumatic System Problems
Mercedes-Benz 300SD Problem
Visitor, 1993 Mercedes-Benz 300SD, 310,000 mi
Close assist never worked since I had the car. We are in the process of replacing all of the components including the two vacuum pumps