I think your door locks are controlled by two separate vacuum lines, one for locking and the other for unlocking, each line has a check valve that stores up vacuum. The unlocking system is working okay. But you have a leak in your locking system.
The color code for locking is yellow with a red stripe. Try to find this hose in your engine compartment before it goes through the firewall. It should have a check valve. hook a vacuum pump up to the yellow with red line and try to create vacuum, thus locking all the doors. If they all lock,then your leak is before that point.
If no doors lock than you have a broken hose on the way to the drivers door or in the drivers door lock diaphragm.
If some doors lock, then the leak is in a line on the way to the first door that did not lock, or in the lock diaphragm of that door.
I would take the door panel off of the first suspicious door lock, and unhook the yellow with red hose from the pump diaphragm. I would hook up a vacuum gauge to the hose and see if it get's vacuum when you apply vacuum to the hose in the engine compartment. If it does, and the vacuum holds that is good news because it means the vacuum system is okay up to that point.
Replace all the hoses, and hook a vacuum pump up to the hose going from that diaphragm to the next door. See if you can create vacuum and lock the doors. If so you have found your leaky diaphragm. Check the connections, and check the diaphragm itself with a vacuum pump to make it activate.
Once you see it you will see how simple and easy to replace it is. It may have a part number on it.
This information is from a 123 chassis manual, and prior experience with a broken vacuum line on a W116.