The door lock system on a W123 is controlled entirely by vacuum. It is a system made of many parts and can be complicated to diagnose properly if you've never done it before, and especially if the system has been left to deteriorate for some time.
The first step to fixing the system is to properly diagnose it. Vacuum components, such as door and trunk lock actuators, can be quite expensive so replacing only those that need it will save you time and money over throwing parts at the system in an effort to fix it.
This guide will walk you through diagnosis of the vacuum door lock system on an early style W123 with the four diaphragm door lock vacuum actuators. However, the technique applies to later models as well; the actuators work the same despite looking different.
Note - if you find that part of your leak is due to old and cracked diaphragms on your early style door lock vacuum actuators, see the repair guide under the Mercedes W123 repair section to learn how to fix these without replacing the entire assembly.