the car in question is my daughter's 1980 300TD. She has had it a little over a month. We have been working on it as we go, it is her daily driver.
I am thinking that the vacuum line may have been cracked, perhaps that is why my daughter reported intermittent failures. The only fast bandage I know of for that is gap-filling ACC (super glue) over the joint while it is held closed; then toss baking soda on the glue to turn it into a "weld bead"- the baking soda instantly freezes the ACC. Once a small bead is built up around the break, wrap with duct tape to keep the joint from vibrating apart. Beats towing the car, and the main vac line to the booster will have to be replaced in any case.
I believe we both have the same type of vacuum pump. Her car was built 10/79, mine was 01/81- but neither one may have the original engine. Lots of years and miles between then and now!
I first did a repair with the ACC/ baking soda about 20 years ago- to a broken vacuum connector. It actually holds up really well. But since this is a Mercedes, the proper repair is appropriate, and the ACC is only a temporary measure.