Faulty master brake vacuum line, this thing:
http://catalog.peachparts.com/item.wws?sku=W0133-1627740&itempk=86596&mfr=Cohline&weight=0.30
Part #: 12343007429
- I tested booster again, it held vacuum indefinitely (phew)
- only thing left was the master vacuum line - so I swapped it into the '82 - and the same slow vacuum build up was observed on the gauge as on the '85.
Ordered a new master line this morning on a hunch, just picked it up, installed and presto...fixed.
Thank you Brian and to all who spurred me into thinking more systematically and not making any assumptions.
The old line looks perfect on visual inspection. No visible faults/cracks/leaks/loose lines/ whatsoever. I must have knocked it just right while doing brake work this week.
-dd
PS It was really, really helpful to have the same car next to me to make instant comparisons
