The dash does not need to be removed to check or R&R the vacuum elements (servos). Removing the passenger airbag is easy enough, just be sure to disconnect the battery first. It is held in by one bolt that goes up thru the kneee bolster. I just got done replacing three bad vacuum elements out of the six on my 420 SEL, two of the bad ones were on the recirculate door just in front of the passenger airbag and the other bad one was the defrost element, the hardest one to get at. A real pain but much better than pulling the dash, did that once on a 380SEL with no airbags, nevermore!!

If you've tested all the vacuum elements at the switching block like you say and they all held, I would guess they are all OK. I would look elsewhere for your problem, like the pushbutton controller for example.

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Chas

Had a 380SL, 450SL, 380SEL, 420SEL, 500SEL

Got a Euro 500SEC right now.