I have an '88 Caprice I've owned for 20 years. When I bought it, it had the vacuum heater valve replaced with a manual valve. Since I've been fixing it up, I got the correct vacuum actuated valve again and installed it. It seems to work correctly, it opens and closes when I move the temp lever back and forth on the dash. However, the only way to not have full heat is to have the temp lever all the way over to the cold side. I'm not familiar with how these systems actually work. Is there some kind of mix door that may be stuck open so air flow is always going through the heater core? I haven't had much luck finding any information related to them online. Any help would be appreciated.