1972 Chevelle A/C question - diverter door operation

[RPOLS3]

I am having an issue when I turn on the A/C in my car - the door in the firewall air conditioning box that diverts the air to the top dash vents is not opening to allow the air to flow properly. It blows out of the heater discharge vent on the trans tunnel. I assume I have a vacuum issue since these doors are vacuum actuated (I think) but I took the dash pad off and some of the ductwork tonight to take a look around and nothing appears out of sorts, all the lines are in place and in good shape. I can open the door by pressing on it - so it is not stuck. A/C is blowing cold and otherwise operating normally, the diverter door just not opening/actuating correctly.

Any suggestions from others who might have had this issue without having to blow the entire dash apart to investigate?

Thanks in advance.