1978 Chevrolet Corvette Air Condition Problem:

I had this same problem with my 78. If your have vacuum going to the control panel and the control panel is plugged in correctly to the vacuum hoses, it is either a hose problem or the actuator motor used to open that particular door is no good.
I would get some vacumm tubing and replace that first since it is cheapest. You will have to take apart the dash by removing the radio, the guage cluster and a few other parts. Make sure not to confuse the hoses as each vacuum motor must coincide with a numbered "port" on the control panel. This is a pretty big job, so don't start if you don't have a full day to jack with it. If you replace the hose and it still won't work and the control assembly is hooked up to the correct hoses, try changing the actuator motor. If you need to know which hose goes where on the operating assembly, let me know and I'll put it up. It is nice once it works correctly. I had to replace everything though, including the contrrol assembly.

Hi! jhr1992 You need to get the vacuum system checked. The A/C is controlled by engine vacuum. Make sure the vacuum hoses are connected under the hood expecialy the one feeding through the fire wall to the AC Control. You should not hear any vacuum leak sound from under the dash. Sometimes you need to pull the control and just do some repair or replacement.
InMrFixIt