thanks, you just gave me the clue that may fix your truck.
The elbow that screws into the regulator cover and connects the FCSV with vac hoses to the regulator, should be only screwed in about 4 threads, and remain ONLY finger tight.
The threads on this elbow, if screwed in 'normally tight' will hit the metal part of the diaphragm internally in the regulator and prevent proper motion of the diaphragm.
This is ONLY to be used to report flooding, spam, advertising and problematic (harassing, abusive or crude) posts.