1978 300SD with a double diaphragm vacuum pump.
First our symptoms: The engine would not shut off with the key, the brakes had little boost and became rock hard quickly, door locks worked but very slowly, the trunk lock even slower and the fuel door slower yet.
Diagnosis: I began with a MityVac hand held vacuum pump bought at a local retail store. Then I disconnected the line from the vacuum pump to the brake booster at the booster end and applied the MityVac to draw a vacuum and got very little - about 3.5 in. Hg, it should be around 19 in Hg. This indicated the vacuum pump was in poor condition and in need of a rebuild. Note that on this car there are no branch lines off the main line from the vacuum pump to the brake booster. Had there been I would have disconnected them and plugged the connection at the tee in the booster line. I began the vacuum system diagnosis here because if there were no vacuum from the pump it would have to be rebuilt anyway before I could diagnose any other leaks.
Repair: To rebuild the pump I ordered the vacuum check valve repair kit, the diaphragm kit and the vacuum pump gasket. A very handy resource is the genuine MB repair manuals that give step-by-step directions for jobs. To remove the pump required removing the radiator and fan, which are not difficult jobs. Then the vacuum pump was removed, cleaned thoroughly, the engine block area cleaned and the repair kits installed. (A handy tool for locating the bolts on the vacuum pump is a mirror; at least one of the bolts is quite obstructed.) This also is not a difficult job and is straightforward. The only item I had to modify slightly was the pump to block gasket, it did not have two holes for the alignment pins on the pump. A paper punch hole was perfect size. Both diaphragms in the pump were torn around the center hub about one and one-half inches. The pump was then reinstalled and everything else was put back in place.
Result: Everything on the vacuum system worked as well as it had ever done. The climate control works as good and quickly as it ever had. We had forgotten how well it responded because the loss of vacuum had been somewhat gradual. The car is a delight to drive.
I have continually found that this car is very straightforward to work on. Things are done in a logical manner and it is imperative to read the repair manual and do the job the way it says. Short cuts lead to backtracking and redoing. As the saying goes, "There is never time to do the job right, but there is always time to do it over". I hope this information is useful to others who have vacuum work to do.
I would like to thank the forum for all the information and bolstering my courage to tackle the job. The cost was less than $150 plus my time (about 5 hours) as compared to an estimated $700 - 1,000 at a service center, but the biggest benefit is the knowledge I gained doing the project.