I've got a '92 300SE, which isn't the same chassis as yours. However, knowing how German engineers like to stick with a theme, I'll bet the line in question is for manifold vacuum assist and it probably runs forward in the car.

On my car, that line wouldn't hold vaccuum either, and while the car was running, or when the key was in the ignition after running, the pump would cycle constantly.

I used ALL-DataDIY to diagnose the problem - if you don't have a membership, I HIGHLY recommend it - $19.95/year for each car model.

Anyway - my line ran forward to a plastic manifold device that feeds vaccuum to several places - A/C, transmission, etc. That $47 manifold was the source of the leak. Replacing that part solved that leak. Curious that it wouldn't hold vaccuum, as it has no moving parts and was in typical German fashion designed and assembled like the proverbial Sherman Tank. I took my old one apart and could find no issues, with vaccuum it doesn't take much of a leak to cause a big problem.

If you know that the line doesn't hold vaccuum, then you also likely know that you should not necessarily assume that what the hose connects to is what leaks - always remember to test the hose itself, plus plug the hose back into the pump and test from the other end to check the valves in the pump.

As for the signal to the pump, I'm afraid I don't know the answer there, but I seem to remember that the design of that signaling system was changed with the release of the new W140 chassis.

Before you worry about that part of the system I'd find and repair the leak in the clear-line circuit and then pull the power to the pump for about 30 seconds. On my pump, certain systems will be taken permanently "off line" if they exhibit excessive demand for air or vaccuum on a repeated basis. I don't know if the 129 pump has this feature, but pulling the power resets the memory for the off line systems.

Hope this helps.