if its the base a/c system and NOT the elect a/c....then look for the vac line entering the firewall about a little toward the passenger side...just behind the intake. Somewhere right in that area will be a vac line thats headed to the cabin. They used to have a T there, and check valve to go to the cruise control too.
IF that is ok and has good vac, then the back of the selector valve has been known to leak. Its enough to hear. take that apart and there is a 'fix' that someone posted here recently.