hey i have a 2000 gmc jimmmy slt with automatic electironic control hvac. i have read threads before but each one is diffrent and i been working at this issue one step at a time but it hasnt helped. when i start the truck at idle i can switch some of my vents but not all of them and it mainly comes out of the dash vents. when im driving the air blows at me when i step on the gas the vacum from the engine sucks it back and causes the vents not to work right til i let off the gas. ive replaced the vacum blend door actuators,newer electonic control vacum port in the dash,checked the vacum lines in the dash,and added a new vacum line from the engine to the vacum resivour and still nothing has changed. any imputs to this mystery would be appreciated

 

can no body help?