1980 300SD Shut-Off Valve Advice, Please.

[Abner Mality]

My 200K 1980 300SD has been parked for three years due to a vacuum pump failure. I have been driving my 420SEL in the meantime, but it is coming up on time for a timing chain replacement, and has developed a few other irritating habits such as a growing promiscuity with any old gasoline pump that it can find along the highway.

I had parked the 300 because I grew tired of having to manually shut off the engine, and was too busy at the time to attempt to make the needed repairs. Now that I want to go back to driving the diesel, I have a few issues. Though I have replaced the vacuum pump, and it is drawing (pulling?) 16 in/hg of vacuum, the engine still will not shut off at the key. Is there any sure way of testing the engine shut-off valve before I spend the money to buy a new one?

Also, the engine runs rough at idle with lots of vibration. Some of that may be related to leaks in the injector return hoses from their sitting and drying out, but I have ordered replacement hose, and will install it as soon as it arrives in a few days. So, that leads to another question: Whether or not I end up changing out the shut-off valve, and after replacing the injector return hoses, what should I do next if the rough idle continues?

Thanks in advance for any suggestions.

UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE UPDATE

It is now Friday afternoon. I had forgotten that she who must be obeyed had a Dr.'s appointment this morning, so I did not get to do any troubleshooting on the engine shut-off valve until after lunch. The good news is that, when I applied vacuum using my hand-held vac pump attached to the stub on the shut-off valve, the engine shut off as it should have ten times in a row. I was careful to watch the guage on the pump, and the engine was shutting off at between 8 and 10 in/Hg; mostly leaning to the lower range. I had reported that the engine vac pump was pulling 16 in/HG, so my conclusion was that there must be a vacuum leak somewhere in the system that is causing the shut-off valve to get less than the 8 to 10 in/Hg that worked it with the hand pump. The first things I checked were the relatively small check valves that attach at different legs of the 4-way rubber vacuum manifold. Both those check valves held good vacuum with no problem. I then attempted to test the other three check valves in the vacuum line leading to the brake booster. These consist of three larger check valves -- two in-line ones and one that comes off a tee and is an "elbow", 90-degree type. All of these check valves are attached to the thicker and much more rigid vacuum line, and I cannot, with reasonable pressure, get the lines off the ends of the check valves. There seems to be a great possibility that I could easily break one or more of those check valves if I get too enthusiastic about pulling them loose. Next question: How in blazes can I coax those check valves out of the hoses so that I can test them, AND, can anyone tell me of any other lines that I should check and the proper method of doing so? Thanks again for any help or suggestions.