I am now at this point

[ 6.) Locate the vacuum "tee"on top of the regulating valve on the injection pump. There will be a black vacuum line coming off the tee going to the vacuum modulator on the transmission. Remove this line. Apply vacuum to this line with a hand vacuum pump such as a MityVac. It should hold vacuum for at least 5 minutes, if not longer. If not you should move down to the vacuum modulator itself and check vacuum directly at the modulator (with all lines and connectors removed.) This will determine whether you have a leak in the line or connectors or in the modulator itself. ]
The black line holds no vacuum at all. I see this line goes into the firewall and I’m not sure where it goes from there.

Can anyone tell me more about where this line goes and the location/description of the vacuum modulator? It would be nice to have a better idea of what to look for before I start feeling around the bottom of a diesel benz.