On my first road trip with my 2000 1500 Express, I noticed the air flow from the vents would diminish over a 4-6 hour drive (max a/c the whole time). Turning the van off during a gas/bio break seemed to help restore it. A friend thought it might be a slight vacuum leak? The fan 'noise' does not change, just less air flow from vents. Changing the settings while driving didn't seem to help, only a shutdown for 10 minutes or so. Any thoughts on where to start looking?

 

the vacuum controls the mode doors not the blower motor...I was thinking more bad resistor or wiring harness to the resistor. there was a recall for melted resistor harnesses for the pickups of that year.

 

The blower does not slow down or make less noise, so I am thinking the "mode doors" are moving? I don't feel air start to come out from other than the selected vents. How hard is it to access these vacuum lines?

 

the mode doors can't be moving if your not getting airflow out of mode door position

there is a vacuum line in the passenger footwell that gets kicked off. pull the carpet back near the dog house and inspect. the vacuum hose and actuator are in that area, behind the glove box on the hvac housing.

 

Do the mode doors not have a natural position? Like a brake caliper piston....if the vacuum bleeds out won't the door return to its natural position? Whether that is open or close I am not sure.

 

sounds right, the vacuum motors probably have springs that move a diaphragm when no vac is present