There is a vacuum valve that has a hole in the line to it.
The default mode of the life support system is to blow out of the defrost
vents. It takes vacuum to get the system to blow out of any other vent, so
you are describing a condition which there is no vacuum control.
On my F150, the valve is by the battery. There is a line from the manifold
that provides the vacuum, this line is compromised. Well, there are two or
three lines, and I suppose the line after the valve could be damaged so that
there is no vacuum left to operate the control module that the vacuum
operates. The valve is very small -- 1/2 inch, max -- and I think it should
be blue and a neutral color.
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