JDL,
On mine since cleaning,painting,new gasket etc..once i release the lever from its locked position mine will pop open abought a 1/4 inch on its own, the linkage dosnt exsert a lot of pressure to open the vent it is a pretty easy push to open it fully.
that is what makes me think the vent itself is stuck to it seat somewhere around its perimeter.(flexes but will not open)

1952 Chevy Shortbed

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If It Aint Broke Fix It Till It Is!