Cowl inlet valve on A/C trucks

[chewed 71'er]

I recently replaced the cowl (plenum) air inlet valve in my 71. It fits good and closes nicely, but will not open on its own. Removed vacuum line from diaphragm, but had no affect. Is the diaphragm return spring weak? Is there an external return spring missing? This is the first time I've dealt with this setup, looking for ideas.