Need help with vacuum lines

[KCKC]

Hi Guys and girls. As said, i need some help with the vacuum in my 83 K10 diesel engine. What is the problem? Changing the lever in the dashboard unit does not give any change in the aitflow. In no matter what position the air flow comes out of the lower air ducts on the floor. I have been checking for any leaks, but could not find anything. When i put the lever to defrost position and rev the enigine, the defroster door opens. when i release the throttle the door closes again. I tried to locate the connenction to the intakemanifold but could not find a connection. So where does it get its vacuum from??? Please let me know. Cees.