Check the vacuum lines. It sounds like the yellow vacuum line from the defrost/dash flapper came loose/leaks. View attachment 470276
The line comes off the back of the HVAC switch and if you look up under the driver side of the dash you should see a vacuum canister. It should look like this. View attachment 470278
I just went through this on my new stang.
alright...took the thing apart and found a leak on the HVAC control side of the vac line, i just haven't isolated it yet. everytime i push down on the plunger of the canister i hear air coming from the controls side.
i assume the red, blue, and white lines are also vac lines, but how does that whole assembly operate? there also is a nipple that doesn't have any line on it. is that how it's supposed to be?
When you push on the plunger you should hear air in the cylinder.
Here's my thread on my problem. Post #10 has what the lines should get vacuum. It also explains how the switch controls the vacuum flow. You may need to disassemble the switch like I did and clean it up good. http://forums.stangnet.com/showthread.php?t=634859
There is a nipple that doesn't have a line on it. The vac lines are as follows.
Red/blue are for the upper/lower ducting flapper.
Yellow is for the defrost/dash flapper.
White is for the recirc(max A/C) flapper.
Black is the vacuum supply from the engine.