Weird Vacuum Problem: Opinions and Wild Guesses Welcome
 

This, in effect, replaces an earlier post because of a new twist (i.e. things work fine then they don't, etc.).

I had both pressure converters replaced to address a poor acceleration problem. The acceleration has in fact improved.

But, when the car has sat for a few hours or more, there are two (I assume interrelated) problems I observe:

1. No air from the center vents even though temp wheel set to full cold position.

2. If I accelerate hard and then take my foot off the pedal completely, there is a “thump” in the transmission.

Neither one of these problems was present prior to replacing the pressure converters.

Interestingly, after I drive for a while, both problems go away: I have air from the center vents and the transmission smoothes out. I imagine this rules out the possibility that something is not attached or attached incorrectly.

What kind of vacuum problem could this be? Perhaps some unintended consequence of how the new pressure converters work or perhaps a leak somewhere?

Here's a wild guess, a very small vac leak, God knows where (one of the reservoirs?). But it takes time to catch up with the leak, then you don't notice it because it's not there after driving.

That makes sense. If so, I wonder: is it just coincidental that the problem developed when I changed the pressure converters? Or did changing the pressure converters cause the leak to develop? OR did the previous pressure converters somehow mask the leak?

Those are very good questions, hopefully you can get some good answers.
Good luck.

Time to break out your MityVac and find the answer. Most kits come with plugs so you can much more quickly isolate the issue.