79 300 A/C Question

I am having an issue with my A/C. All the mechanical parts are working and the system is charged. I have vacuum working in the system.

When I use MAX A/C the compressor cuts in and the car cools. When I use "A/C" the compressor cuts in and the heater box cools to touch, but it blows warm air only. As well, warm air blows when I use no A/C vent. I have a feeling its the controller, which uses vacuum and is located under the dash on passenger side and attached to the heater box with 2 small bolts. I know there is vacuum to the controller switch and the actuator moves freely when I change the temperature control. Sometimes when I slide the temperature back and forth a few times I get a blip of cold air coming through but it isn't sustained...sigh!!!

Anyone have any ideas whether this is this controller issue? Could the blend door not be opening due to being stuck? How do I find this blend door? Can I get one of these vacuum controller thingees? Does anyone have the part number for this part (and the correct name? some have called it a heater control valve but any I've seen look very different).

Thanks for your help!

First thing I'd do is check everything, hoses/tabs,etc. under there to be sure nothing is cracked,broken,etc. also check for any debris or crap clogging or jamming, before spending $ for a part.