The early years of the square body had the blower running to "Pressurize" the cabin to keep unwanted dust from leaqking in like it does with the pre 73 models.
I remember a guy I worked for many many years ago bought a brand spanking new 73 GMC and He complained about that and the dealer did a tech check and that is what was passed down.
SO,if You want unwanted dust settling into Your nice Chevy/GMC square bodies then shut offt the blower completely.
After I found out about this I now roll up all the windows and run the blower on high when traveling dirt roads.thats all.
