Extra vacuum port on HVAC control

[tucsonjwt]

I am slowly putting the interior of my 83 C20 back together and I seem to have one "extra" unused vacuum port. The vacuum diagram in the shop manual uses different color striped vacuum lines than I have, and the colors I do have seem to go to different places.

I had two loose vacuum lines after reinstalling the HVAC box on the firewall, and based on the length of the lines I hooked them up to the open vacuum ports on the heater box that seemed to be appropriate (one on the top of the HVAC box which had a plastic vacuum connector attached to a vacuum line going inside the box, and the other line had a right angle type of connector which attached to the vacuum actuator on the driver's side end of the box.)

The open port on the vacuum connection on the back of the HVAC control in the dash is on the top. Is it possible that I have an unused port on that valve from the factory? I don't see any other loose vacuum lines under the dash, so I don't know if there should be something in that open port - maybe a plug of some sort? I got this HVAC control from LMC many years ago, so I wonder if I got a generic control (maybe also for suburbans with rear ac?)

Thanks

[andyh1956]

The whole system has to vent somewhere, that could be the place.

[tucsonjwt]

I hope you are right about that venting, since I have no unattached vacuum lines under the dash.