Just tossing this out as a fwiw.....on my 90 it wasn't one or two big (detectable) vac leaks. Rather, it was a collection of really small ones from all the rubber parts in the PCV sys and the dual connector at the bottom left of the T-body....just a thought.
I used small zip ties to correct the T-body & two larger Z-ties at the PCV holder & it's top connector. I replaced both 90* el's @ the I/H's and made sure to re-tq the plenum bolts after a run cycle. I know that was a lot of redneck engineering but alot of that rubber is NLA from GM. Even the new stuff I bought when it was available didn't last long at being tight. I guess our underhood temps must kill the rubber parts quickly.
Tom