Mystery vacuum leak

[Joey41]

So here is the story. When I accelerate to get on the highway my ac goes from the upper vents down to the floor vents and then as I get up to speed on the highway the air comes back to the top vents. I have been told this is a usual sign of a vacuum leak, but I cannot find it. Have checked all vacuum lines on top of engine. Just had them all out and inspected them while doing upper & lower intake mani gaskets. I do hear a wooshing noise when I apply my brakes which my Dad says means my brake power booster is out - could the power booster be behind the vacuum leak? Just curious if anyone else has experienced similar situations. Searched but did not find anything.

Oh, it's a 97 Camaro, v6, automatic with about 240k miles on it...

Any help is greatly appreciated!

[Joey41]

Re: Mystery vacuum leak

Just searched another way and found one similar post...

I should mention that my check valve located passenger side top is in good shape, and that I replaced the line going to the vacuum resevoir about 2 years ago.