I just went through this with my 1983 300 CD. There may be some slight differences but I think they are about the same. The first test that I did was to check the check valve, yellow, near the fire wall in the engine compartment that has two yellow hard plastic lines going through the fire wall. Take the check valve off and see if it working. Then with the mighty vac pull a vacuum on each line and see if they will hold. One of the lines goes to the reservoir which will take quite a bit of pumping before it has a good vacuum on it or if you can't pull a vacuum there may be a leak. (mine was fine) Do the same with the other yellow line which goes to the locks. (this would hold a vacuum when the locks were open and not when they were closed. Therefore, I assumed that it was on of the actuators, my gas flap lock had failed long ago so I suspected this was the culprit.)

I unplugged the gas flap actuator and plugged it off and the other locks worked better but it would still not hold a vacuum for more that 30 minutes. I then decided to buy actuators for all the other locks, (this was not too bad since I only have two doors). I replaced all the actuators including the gas flap and it works like a charm and holds a vacuum overnight. I don't know exactly which actuator was causing the slow leak and if you want to spend more time tracking each one, you can probably avoid buying all the actuators. I figured that I might as well replace them all.

If necessary you can access junctions in the vacuum lines underneath the carpeting on the front passenger side (at least on a 300 CD). I pulled my carpet but did not have to actually disconnect the lines to individually test each actuator. (the way mine were run made it look like it would be a bit difficult to disconnect, test and reconnect, so I opted to replace all actuators.)

Partial removal of the door panels was fairly easy. First remove all the hardware such as handles, chrome decorative plates and there is a plastic insert in the door handle that has to be removed to remove a larger screw. After all the hardware is removed, then with a flat screwdriver or I used a thin prybar, you just "pop" the holder clips directly out. (you may want to buy some new ones since they get brittle over 20 years and a few had to be replaced.) I did not have to completely remove the door panel to replace the door actuators. I tried but the clips that hold the panel on the top were bound too tight and I was afraid of breaking something and did not need to remove them entirely.

The trunk and gas flap actuators were fairly easy to replace as well. I also purchased a manual from **************.com which was very helpful in giving me the confidence to removing the door panel. You should also check out thomaspin's (he has several posts) web site which shows pictures and gives a great overview of removal of the door panel (as well as many other very helpful things relating to working on mercedes).

Hope the helps. I am sure that a search will also turn up several other helpful posts.