I have checked for a vacuum leak; looked for cracked lines; loose connections; can't hear any hissing & also checked with starting fluid...could not find it. When I turn on the air (not the a/c), it blows through the defrost vents. When I turn it to A/C, it works fine, it will blow through the vents. Any suggestions? Thanks.

 

Try pulling the actual climate control head unit out and look at it.

The vacuum lines attach to the back of it as well as the cables that control the heater box flaps.

I believe the T line back near the pass rear of the intake with what looks like a little cansiter(believe its a check vavle actually) in the line is the one that supplies vacuum into the car for the HVAC controls.

later
Jeremy