I think you alluded to this: IF you have a large vacuum leak down stream, your vacuum gauge may read 0 even when measuring at the main line. I disconnect all possible leak sources and plug the ports with plastic golf tees. (Wooden Ts get stuck in rubber.) You may find several small leaks in the branches going to the various cabin systems.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.