Looking for advice

[John300SDTurbo]

I seem to have developed a vacuum leak but I'm unsure where to start looking. I'm hoping one of you can point me in the right direction. Here's the deal..... had a rather cool morning yesterday and so I spun my ACC dial to full heat to let it warm things up a touch. All was well until I got to work and turned off the key switch. You guessed it! The old girl kept right on running. Realizing that the ACC dial was the only thing I had messed with I turned the key switch back on and spun the ACC dial back to full cool. Turned the switch off and bingo, she shut right down. I think there is a control valve against the firewall beside the battery somewhere that controls water flow to the heater coil (if I understood some of the stuff I've read), is this control valve vacuum activated and could it be the problem? If not, any suggestions on where to start looking? Any advice is greatly appreciated.