Lux...where the air goes is determined by vacuum. A few people have mentioned checking the vacuum. Until you check the vacuum, it is very tough to help. Vac is the first thing to check and the most common problem.
You already are pretty well into this. Engine compartment: Check the hose going to the black tank next to the brake booster. I had that same exact problem. I found my vacuum hose came unhooked outside at the vacuum tank.... if the hose is plugged into the vac tank..run the car and unplug the hose..you should hear a definate vacuum refilling with air when you pull the line.
Here'* what I found. The large tube is the brake booster line, the small...the line that goes to the vac tank....looks disconnected.