Turned on heat this morning and noticed legs were getting cold.Tried to get heat to come out on floor but only derost will work . I had thought this year truck no longer used vacuum pump system but it looks like I was mistaken.Brakes are hydraulic boost so what is left for vacuum to operate.

 

I am almost positive that the gate for the directionoal "flapper" (sorry to be so technical) is vacuum. Defrost is the normal postion when you loose vacuum. You can hear the air in my '04 so I thikn yours would be the same.

 

Sounds like you lost your vacuum. I guess what I would do is start at the vacuum pump and check the vacuum lines for leaks. Just start at one end or the other and look at all of them. You will probably find a cracked or broken line somewhere. The HVAC systems "default" to the defrost when there is no vacuum.
Tom

 

https://www.dieseltruckresource.com/...threadid=30705

 

Check your **** first, as they can and do strip out. I have ran across this before. Goodluck,, Rick

 

checked vacuum at firewall connector had 27 Hg. .pulled switch out of dash ,checked at switch and had no vacuum.pulled a vacuum with mightyvac at firewall connector going in to switch and it held .now I need to find out whats between firewall connector and dash switch.