67 A/C cab

[Andy4639]

You have to remember that the diaphragm's all look alike but depending on what year model they came from will determine how they operate.

Adding vacuum either opens are shuts them. There is no way to tell by looking at them that I know of.

On my 71 C 10 I took the cowl vaccum diaphragm out & the flapper all together. I permanently sealed it off. I also swapped the passenger side kick panel back to manual and I leave it open all the time. This in effect does the same thing as in a late model vehicle on recirculation air. It keeps the cab air recirculating to make it colder without adding outside air.
It works great and if you need some fresh air just open the drivers side kick panel are a window. It keeps the truck nice and cool during the middle of the hot summer no matter how hot it gets.