OK, I checked my engine compartment but mine doesn't exactly match your description. There is a vacuum line off the manifold, as large as the brake booster hose, that feeds a miniscule plastic hose going into the firewall. After locating the check valve and fiddling with it, I managed to make it work again. Now, I don't notice the hissing sound inside the car. I took it for a test run with the fan and A/C on and there is no longer any interruptions in A/C when I'm on the boost. I think that I will buy a new check valve anyways since this one is a bit suspect. Thanks to all for the input!