Pop the hood, look at where your A/C lines come out of the firewall, in between them is a vacuum line running to the driver side fender apron where it connects to a vac. tree. Somewhere that line is broken or kinked. Best bet is to go to the parts store and replace that line from the firewall forward.
If that doesn't fix it, let us know and we can advise you further.
I agree with The Mustang, I had the same problem and it ended up being a broken vaccum line by the firewall. Bought some connectors at the store and fixed the line.
^^^all these guys are right. though, the connectors from the parts store seem to have been engineered to crack after a few months. i got fed up with buying them and just used electrical tape