Hi, i have a 97 B3500,it’s been bugging me for a long time now, when I step on the gas, the vents all switch to defrost, somewhere along the vacuum lines there was an issue, I tried changing the check valves, and that didn’t help, so I came up with a new plan, when I was at the scrap yard, I picked a vacuum ball from a similar van, and installed it mid line just after the brake booster, just added a T in the existing vacuum line, and that was all I needed to solve the problem. Now the vacuum has sufficient pressure without going to defrost setting,,




My vents all work perfectly now.I took some pics to show how I did it, if you have any questions please let me know,,,take care,,,,

 

Nice find.
Yes, that part goes bad all the time and is hard to detect if it is an issue.
You can seal a suspected one with silicone on the seams and then the spray it with bedliner. That can work if you are super cheap LOL Don't ask me how I know...