I have a 98 F150. Last week the fan blowing in the cab kept moving from the “Panel” area where I had it set to the defroster area. I noticed it would do this when the engine vacuum was low (acceleration) so I assumed I had a vacuum leak. After telling my brother about it he said his Dodge done the same thing. He found that it was a check valve in a vacuum reservoir. I thought this made more sense than an outright leak. Can someone tell me where this reservoir is?

 

May be a blend door actuator going bad.... Post in hvac for a better/more response??

 

Go to the HVAC forum and SEARCH for threads containing "vacuum" and "defrost"

 

The vacuum reservoir and check valve is probably on the passenger side fender apron. The reservoir will be a metal/plastic can/ball with two vacuum sources going in to it and a vacuum line going in to the firewall (goes to your heater controls). The two sources provide both closed and open throttle vacuum to the reservoir, and the check valve keeps the vacuum in the reservoir.