I have a problem with luke warm air coming from my heater. I just recently found that I don't have vacuum at the heater core control valve, so the valve is always closed (no water circulating in the core). I manually opened the valve and now have heat. The vacuum line is coming from the firewall. I assume I have a vacuum leak under the dash somewhere. Does anyone have any suggestions where to start troubleshooting and what to take apart under the dash? Thanks. Ken