Tom, I looked under the passenger seat but all I saw were motors, etc.. nothing that looked pneumatic. Maybe I didn't look hard enough but could you describe it to me.

I was able to confirm the hissing from the passenger front door when the lock was in the up position. I went away when I pushed it down. While fiddling with the locks trying to get them to come up, all but the driver's were "holding" themselves down, I was pulling on the rear drivers and all of them just came right up.

Just to increase my understanding, I'm assuming there is a switching mechanism in the driver's front door right? Some how this must tell the pump to come on and also redirect the vacuum? Or does it tell the pump to go the other way? Or is the vacuum switched on the pump?

If I can locate that manifold and do some vacuum testing, that would give me great confidence choosing that parts to replace.

Thanks for the help!

Frank.

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'82 300SD