on my 93 dually. i am having issues with the vents not changing from derfrost to vent or floor. stuck on derfrost. does anyone have a diagram of vacuum lines in dash an outside at firewall. as it feels like it has no vacuum.

 

Sometimes the doors in the HVAC box will seize up. take off the glove box door,
then you can get your hand on the vacuum linkages, check for frozen linkages

 

Check the small hose at the brake booster check valve.

 

I had that issue..
was a bad check valve.. but I also had brake issues as well..

 

The main control assembly on the heater/AC control can warp from heat and age. Then they crack open and do not allow the vacuum to be re-directed properly.

But start at the booster, and work your way along all the lines to the interior. You might find a small hole in one of the lines, from rubbing against something...I had this happen to me.