Heater - Vacuum Drop

[rexus31]

I was cruising my '65 300 'vert this evening and noticed a 2-3in vacuum drop with the Heater on. When I turned it off (and heard the fresh air door close) vacuum went back to normal. I don't recall ever experiencing this. Any ideas?

 

[commando1]

Stick your head up under the dash and go thru all the buttons.
see if you can hear a leak.

 

[cantflip]

With the vacuum harness off the control panel... you can use a vacuum pump to find the leaky element(s)... more accurate and less painful