With your hand vacuum pump connected to the solid yellow line, with no stripes, you can continue. Make sure the driver's side door is closed, and turn the key to the lock position. Pull a vacuum of 10 to 15Hg; all the doors on the car should lock. Wait about 5 minutes to see if the lock side of the system holds. Make note of the results.
With your passenger side door panel removed, and the door left open, you can watch one of the actuators during this process. The door panel removal technique guide is available in the Mercedes W123 repair section. Click here to read that guide. Pictured are the early style actuators. The first picture shows the door in the locked position while the second shows it unlocked.
With your hand pump still connected to the solid yellow line, move the key in the driver's door to the unlocked position. Pump vacuum again at 10 to 15Hg. The doors should all unlock. Retain this vacuum again for about 5 minutes. Make note of the result.