The A\C|Heater|Outside Air is in the cab just fine, but the air is either on the dash\defrost or on the occupant's feet. The vent control from the floor vent to body vent to defrost vent is unreliable.

The occupant's body vent (mid level air) does not always work. When it does not, I can hear a hissing coming from within the cab. If I operate the control several times, it does begin to work sometimes. If I drive the vehicle for a little distance it will begin to work after I operate the control several times. It sounds like a vacuum line is leaking when it does not work.

I have looked under the dash and do not see any obvious issues.

Are there drawings available that will help in troubleshooting?

Can anyone help get me started?

Thanks

Dave