I have an 84 corvette that I purchased 1 year ago. I have replaced the A/C compressor and filled the system with R12 and when I run the air conditioner for around 30 minutes the air stops coming out of the vents. It sounds like the air is going somewhere in the dash. To get it going again I switch over to vent for a few minutes and then back to A/C. It works for another 30 minutes. Is there a vacuum opening the vents that might be leaking?

 

No expert here, but you may find help from my 86 manual.
http://members.shaw.ca/dankai/Air%20...0%20Manual.pdf

 

Thanks I'll take a look.

 

Don't know the '84, but vacuum has been used forever to control the HVAC vents. It usually flows through a check valve - or one way valve that keeps it from bleeding off. That valve is often teed into a tank which stores vacuum when manifold pressure is low. Key is to find the source in the engine compartment and trace it to the check valve. Make sure it's in good shape by blowing through it - you should only be able to get air through it in one direction. The vacuum tank on the '85 and up is in the left headlight well - hopefully yours is the same. Verify that it can hold 10 in of vacuum. If all that checks out, vacuum flows from the check valve to your dash controls. By covering and uncovering ports as you twist the ****, it's directed to solenoids on the heater box. Up until the early '90's, GM usually kept vacuum on the Defrost door at all times using it to keep it close or suck it open. Lose the source and it will stay open all the time and all you get is defrost. The other doors will get vacuum only when they need to open or close. If a solenoid is bleeding off, you can get the symptoms you described. Troubleshooting is easiest by disconnecting the line and using a vacuum pump on the solenoid. Vacuum should hold. If it doesn't you need a new one. Solenoids are usually close to the bottom of the heater box, about the middle of most cars I've owned or worked on, and closer to the accelerator side than the passenger's footwell.

 

: Vacuum problem probably. Not sure about the 84 but… If you have the plastic covers on above the valve covers look on pass side for a 'T' connection. On the later model known to go bad and break up from the heat. Otherwise look near vacuum takes offs where there would be a hose and lots of heat usually near the fittings. Hope some of this might apply.