Diverter valve vacuum motor not holding vacuum.
Been gone for a while once the tranny in my 84 GMC K1500 died. I decided I needed a newer truck, so I picked up an 85 Chevy K10. Same basic truck including a working 700R4, a better running engine (both are LE9 5.0's), and very little rust on the body. It's a little cold-blooded and was idling really rough due to many small vacuum leaks. Most of them were in rotted/broken vacuum lines. I got it from 8" of vacuum to 15", which is still low. I think it's leaking either at the carb-to-manifold junction or at the primary throttle shaft in the baseplate.
But, if I disconnect the diverter valve (AIR system, not PCV/evap decel valve) and plug the line, I get another 2" of vacuum. The valve works, but it won't hold vacuum at all. I can pump it up to about 15" and it leaks down in about 2 seconds. I tried the same thing on my 84 (which is currently being scavenged for parts for this one) and it does the same thing, but it leaks down a little slower.
I don't remember back from when I was a mechanic (many, many, many years ago) if that behavior was by design because of the way that the diverter valve works.
Any ideas? And, please, no "Tear-the-emissions-stuff-off-of-there-its-not-doing-you-any-good-and-its-a-communist-conspiracy" answers. I have to have it for emissions testing here.