I have a vacuum leak when I turn the AC control to Max air. I don't have it any other time. I'm not familiar with the layout of the duct controls and don't know where to start finding the bad line or control.
thanks

 

Difference between MAX AC and Regular AC is the Fresh air vent, it closes for MAX AC to recirculate cab air instead of trying to cool down outside air.
Usually Passenger side of cab behind or next to glove box

On some years there will be a Grey vacuum hose coming out of the firewall next to the Blower/Fan, in the engine bay, it will be with a Black Vacuum hose, they look like wires because they are Plastic hoses
Grey hose goes to Heater Hose Valve, on some years MAX AC also closed this vacuum valve to stop hot coolant from flowing thru the core.

Fresh air valve is always used

Here is a diagram of the system: https://www.ford-trucks.com/forums/a...ntid=517&stc=1

On the right hand side you see the White Vacuum hose coming out of the Control Switch, it splits off to the Grey hose that runs thru to the engine bay and heater hose valve, and white hose continues on to Outside/recirculation valve located inside the cab
They call the valves "vacuum motors"

Each vent door is labelled NV for no vacuum position and V for with vacuum position

Temperature door(Blend Door), is operated by an electric motor, not vacuum.

 

So how would I go about finding the problem? Should I disconnect lines and pull a vacuum or is it more likely to be in the controls?

 

Unplug the fuse for the fan and unplug the AC Compressor wires
Start engine and switch to AC MAX
Turn off the radio, lol.
Listen for a leak under the dash