Heater blowing only thru vents

Hey everybody, My 85 Bronco heater, will only blow air thru the vents. It happened before, and I found a vaccuum line off one of the little cannisters under the dash, however, I can find no lines off now. Any ideas? It does not seem to be getting any vaccuum to the cannisters to make the switch work. I have traced all of the vaccuum lines, and can find nothing wrong. Any help will be appreciated.

Check the lines for any hard bendings or if they are leaking through holes made by friction.