A very common problem that requires precise troubleshooting
Vacuum problems are so common on older Mercedes (especially diesels) that Kent wrote his very first manual on troubleshooting vacuum source problems. 123 chassis door locks can be very troublesome. He has written a manual specifically on this subject. It will require a vacuum hand pump tester and a willingness to learn how the system works and the parts needed to fix it.
Problem & Solution
Common among these chassis:
W123
For more information see related products and videos.