I have a diesel 83, having dropped the fuel tank because of holes I can testify
that I have a fairly complicated vapour trapping system as well - im not
sure if david's 84 is that different from the rest - anyone comment.

My recollection (notoriously unreliable) is that i have the two white
plastic bottles on each side with about a dozen separate hoses total -
i replaced them all with tygon when i did my tank (WARNING - I WOULDNT
do THIS WITH A GASOLINE WITHOUT CHECKING COMPATIBILITY !!)

sounds like you do have some plugged line - good news is this easily
fixed - bad news is if its like mine you have to drop the tank to get
to any of these - i recall about 4 hours of work total.