I have a 1990 Corvette (110K miles) that comfortably passes the environmental sniff testing but failed because it won’t hold vacuum. And, yes, the A/C will shift from the dash to the floor under load. I’ve visually checked for cracks and breaks. I’ve changed lines that look remotely aged. I’ve checked “T”s and junctions. I’ve mentioned this at garages and get the same response you might get when you ask about an intermittent electrical problem: No one wants to deal with it!
I was in your shoes a few years ago, but with the dreaded P0171 engine error code on my 1995 Lincoln Mark VIII LSC. I replaced all vacuum hoses around 2007, and they still seemed OK. Ditto for every other item on the diagnostic checklist. Rather than throw money at the problem, I bought a rudimentary smoke machine and hoped it provided a very obvious solution...
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Yeah, those smoke machines work! I'm the guy that wrote the original question and shortly after I wrote it, I, too, ordered a finely made paint can based all-American smoke machine from an east coast state. It makes smoke with 12 volts from the battery and the pressure is from your compressor but regulated down to 1.5 or so PSI with what looks like the pressure regulator from your propane gas grill. It worked as advertised.
I took off the snorkel to the throttle body and put a piece of plastic wrap around it and secured it in place with rubber band. 1.5 PSI blew that off right away. I then reinforce it with a shop towel and the snorkel clamp -- much better. It probably took about 20 seconds of smoke under pressure to fill the system. Yes! I found my leak. One run of vacuum line ran close to the exhaust manifold and the top looked great but the bottom had a gaping hole in it and smoke was pouring out. R&R and my AC is much more stable now. The old girl runs better, too.
I also saw a small amount of smoke from the ends of the throttle body butterfly axle. I have to think about that a bit....
The Corvette may not have a vacuum leak at all but rather a defective vacuum check valve. It can usually be found in the vacuum line from the intake manifold to the reservoir. It might be a "TEE" fitting but sometimes I've seen it incorporated into the vacuum reservoir itself in older cars. I've found that issue in many different makes and models. The check valve on the brake booster can also cause an issue with vacuum power brakes.
The smoke machine should work just fine there too! But it might be easier to add a dye instead (provided the motor can be cleaned to make finding the leak easier)