Join Date: May 2001
Location: Chattanooga, TN, USA
I'm speaking 69, here, but assume 68 is similar. There should be a vacuum valve in line on the heater hose. It is riveted to the inner fender, and its purpose is to control the hot water, when the A/C is on. See if you have one, if it has been bypassed, or if it works.
You might just have a vacuum leak. The vacuum tube connects to a tee under the dash, on the pass side, and comes tough the cowl. maybe you have a bad tube, or a bad connector.