Its considered normal W123 practice to lock the doors before turning off the engine.
Seriously, I bet you have a leaking diaphram in one of the doors that is NOT the driver's door. As mentioned above, get a Mity Vac, which is an invaluable tool for troubleshooting these system. You can access all door diaphrams from the "Y" connecunder the carpet. The main junction is under the front of the passenger front seat. Slide the seat back and remove the front section of carpet. You'll see red and green tubing connected to "Y" connections. Slide the tubing out of the "Y" and look for cracks. Apply vacuum to the various green tubes and see how well they hold vacuum, or not. Replacing the actuators is simple. If you are adventerous, you can remove the actuators and simply work a balloon or even a male contraceptive over the existing boot and hold it in place with the plastic cap.
Alternatively, once you isolate the defective diaphram, you could plug the vacuum line to that door to stop the leak.