Brian,
Do you mean disconnect the electrical lead to the vacuum solenoid? or disconnect the vacuum lines themselves?

This makes total sense to me. I am looking at the service manual and in typical German engineering speak what you just put into Simple English is how this system works. In fact,I believe Mercedes changed this system for the 87 model. Correction, they changed it for the 90 and 91 350sdl models, and made a modification to the transmission itself

John