I have just begun working on my vacuun system to try and fix my locking system. After some preliminary work I found that my car would not shut off from the ignition.
I am begining to explore more and I found that there are two check valves that are blue and black. I was told by a source that the 1977 would have a yellow check valve. I was wondering if this difference was a change by MB or by the previous owner.

My cruise is also not working and there is a unit on the drivers side wheel well that I was told is the vacuum cruise system. If this is so there is a cable from there to the throttle linkage with a white plastic piece threaded into the linkage. I know its a long shot but this plastic piece is broken and I can't find a replacement. I don't know if that would fix it but I can't find the part anyway.

Thanks
Dave

Welcome to the forum Saildavid. The first check you will need to make on your vacuum system is the vacuum pump itself. The early 300D uses a diaphram style pump, and the diaphram tears regularly on these cars.
Just get a vac guage and check to see the amount of vacuum you are getting. Best to check at one of the small lines coming off the large rigid line. It is also possible there is one or more vacuun leaks. Do the doors all lock when you turn the key? If they do then your vac system is working ok and it is the shut off valve.
If there is sufficient vacuum then it is very likely the vacuum shut off on the rear of the Injector Pump (IP) has failed. Those do not cost very much, you can get them from Fastlane, but to not attempt to replace it without getting very thorough and very clear instructions. This forum will attempt to help with that, but always remember that this is the one part that if not absolutely installed correctly will blowup your diesel faster than you can possibly imagine.
That big cruise contol part under the hood is the servo. It is not real likely that part is your problem, usually the amplifier under the dash goes first, although both can fail. That part is up dash itself on the left of the steering column, an aluminum case about the size of a bar of soap. Bring lots of $$ to get one rebuilt.
Good luck, and welcome to the forum.