1982 300 TD takes a long time to shut off. When I plug vacuum line before the door system check valve the car shuts right off. Opened up driver's door and I put mighty vac on drivers door outgoing yellow & red line and system locked. then on outgoing yellow and green and system opened. Hmmm. Then tied mighty vac to master valve in line and system working fine. Then I tested line just before check valve in the engine compartment and pulled a vacuum - - took a while to pump up the resevoir tank but it held a vacuum. So where is the problem? I checked what the vacuum system is pulling just before the check valve and it is pulling about 3 in.Hg. Is that too low?