I don't believe the AC line in the W126 is green for some reason. Grey and red?
It will have a check valve, so you can just hook the ol mitivac to all the lines going to the passenger compartment and draw a vac. One will be the vac "economy" gauge on the dash if you have one.
If you get a system that moves the flaps as you push the buttons but leaks down, you have some bad diaphrams or swtichover valves (they don't usually fail, but it's not impossible). If it works fine with the MitiVac, you have a bad hose going to the manifold.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!