I have a 1979 corvette. When my heat control lever is on cold, I can switch the air flow from the floor, to vent, and to defrost. When the heat control lever is moved to hot, air flow will only come out at the floor. I can not switch to vent or defrost. The vacuum hose going to my heater hose control valve has vacuum when the heat control lever is on cold. The heater hose control valve has no vacuum when the lever is moved to hot. I would appreciate any help on where to start trouble shooting this problem.

 

I would first check the hot water shut-off switch for proper connection/operation Remove the glove box liner and you'll see the switch on top of the HVAC "box". It should have two vacuum lines connected to it.

That switch can be used in both a normally open and normally closed modes. If connected improperly or defective it may well expose the vacuum that also supplies the HVAC mode switch to atmosphere thus preventing anything else from working.

Different models of that switch were used so I can only describe how it should be connected.

Disconnect everything from the switch. Attach a hand vacuum pump (or a tube if you just want to do it by mouth) to ONE of the connections. DO NOT PRESS THE ACTUATING SWITCH. Try each connection until you find one that holds vacuum.

Once you find one that holds vacuum PRESS THE SWITCH and while applying vacuum find which other connection is now sucking air. ONLY ONE of the other connections should be sucking air. If both are sucking air then go back to step one looking for another connection that ALWAYS holds vacuum WITHOUT PRESSING THE SWITCH AND WITH THE OTHER TWO CONNECTIONS OPEN.

The connection that holds vacuum WITHOUT pressing the switch and WITHOUT blocking the other two connections goes to engine vacuum.

The connection that has vacuum with the switch PRESSED goes to the hot water shut-off valve.

 

Hopefully this will help you.

 

I've read through this thread and there is a wealth of excellent information on the whole heater/AC vacuum system. I find the color-coded diagram excellent! I recently acquired a 1980 coupe and suspect a number of issues in the system. The car sat for over 25 yrs., so I suspect multiple leaks in multiple places. The air only comes out of the floor vents fight now; The temp control, when slid to COLD, never compresses the bypass valve vacuum switch. I read the troubleshooting procedure posted by SwanpeastMike (thanks) and will use that to confirm operation of the valve. I'm not sure where physically the A/C air valve is located (Red hose) or the Recirc interior vent valve (tan hose) of the exterior vent valve (green hose). Most of the hoses are black so I suspect PO did not follow color codes. I did find the blue hose (I'm 1 for 4). Any guidance will be appreciated. Until I identify the location of the various vent actuators, I wont be able to get the proper vent connected to the proper port. I also notice that the selector switch (Off-Htr-AC-Def) is significantly more complex than suspected and likely is nonfunctional, but until I get the actuators into the correct ports and verify that the actuators actually work, I can't troubleshoot the switch. I found the hot water shutoff valve and it holds vacuum, but I'm not sure if it actually closes off the hot water.
Your help on where the other 4 actuators are positioned is appreciated.

 

you might get more answers by starting your won thread.lots of people remember to look t the date of the original post ( unlike myself who fell into this) nd just blow right past it. as for the hoses you probably have to take a lot of the heater ductwork out to see where they start. Or get yourself a hand vacuum pump, hook it up to one line at a time off the heater control and see if any doors move. That would be easier than taking all the dash apart. Start a new thread and you will be given lots of ideas. Good luck and welcome aboard.