Vehicle: 1984 300sd California Model - (older vac system like the Federal)

I have been noticing a heavy vacuum leak lately so I pulled out the MityVac and started testing lines. I found the leak in a line that is connected to the system near the brake booster. I have one line that connects to the check valve on the IP, another line that goes into the passenger compartment, and another line which i do not what it leads to. It connects just before the brake booster and splits into two lines that go to the sides of the car. Any idea what its purpose is? When I plug this line my trans shifts perfect and with this line connected i hear the clunk when downshifting.. HELP

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Does the climate control system function properly? Does the defrost outlets properly close and the dash vents and footwell vents properly open..........or do you get all defrost..........all the time?

One of the lines tucks behind the fuse box and goes to a vacuum reservoir in the left fender. Make sure the reseroir is air tight.

Sixto
87 300D