OK Gearheads, I just spent all day Sunday, (a crime in itself ) removing the dash to replace my heater core. The job went ok,(about 7hrs) but after the last screw was in place, I fired her up and BOOM ! I had all the heat in the world.....thru my dash vents only. (the ones facing the passengers at chest level ). When turning the far right control knob (the one that controls the vent selections) I could not change the direction of the air flow. Mainly, I could NOT get flow thru the defrost vents. A/C compressor would turn on in A/C and defrost position but still no flow thru the defrost vents at the windshield. I checked the vacuum actuator behind the glove box, and it moves freely but there seems to be no vacuum in the hose regardless of switch position. Vacuum hoses from dash switch seem free from obstructions. I'm ready to pull the dash again but what should I look for ? A pinched vacuum hose, Something blocking a vent lever from moving ? A sticky vent ? Everything worked fine before I attempted a job that seemed marginally easier than rocket science. (and saved me 6+hrs labour). Have any of you had a similar experience ?
Any and all replies would be most appreciated !

Cheers, from the Halton Riviera !