The vac will drop to 0 in the operating lines -- vac is applied then sealed -- when the other action is selected, the first line is vented.

If the vac in the supply side drops off with the key centered, replace the vac switch if adjusting the position (it has some back and forth adjustment) doesn't fix it, it is leaking. System must hold vac indefinitely when the key is centered.

Make sure the vac tank under the left front fender is holding vac, too -- the rubber gromet at the top where the vac line is connected goes bad and the vac leaks off. Just pumping down the supply line to the door itself and not the tank won't give you any reserve vac, just what is in the MitVac.

If the tank won't hold, you have your problem. If it does, check the switch.

Peter

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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!