A/C and heat will only blow out of defrost vents. How can I fix this problem? on 1995 Cadillac DeVille

Most of the time the A/C air comes out of the defrost, but sparatically it will come out of the regular vents for a short period of time and then it goes back to the defrost vents. The heat always comes out of the defrost vents. I have tried checking for vacume leaks and didnt find any that was obvious.

Asked by for the 1995 Cadillac DeVille
sounds like an accuator door is binding...Try to operate it by hand...The diaphram inside a module that operates the door may be leaking vacuum.