L31MaxExpress
I'm Awesome
In the past I too have used the United cap. The little vent dome pulled right off its slight press fit leaving a perfect vacuum nipple. However that leaves a path for dirt to enter unless you supply a filtered fresh air vent. My setup closely replicates GM OptiSpark venting.Like 454CID mentioned, the GMT400 pickups don't use any vacuum in their HVAC controls - all electronic. The exception would be that one heater control valve used on some of the SUV models. So if you want to use the vacuum method you may need to choose a different source.
The GM TSB about the distributor vents seems to only cover models other than 400's which is puzzling to me as I can't imagine they use a different distributor. Maybe there's some revision? Anyway, if there are still vent screens in the base of your Vortec distributor, pop 'em out and straight to the trash with them, and then be sure the vent holes are clear. The bulletin is GM TSB 03-06-04-041A by the way, and for whatever reason that one little bit of info doesn't just jump out at you when researching the subject. Here's mention of it by NAPA but they're using it to suggest buying their Echlin distributor cap which has brass instead of aluminum terminals. https://www.napaechlin.com/media/3137/ei002-05-gm-vortec-distributor-issue.pdf
TSB info aside, the vented cap offered by United seems like another way to achieve this.
Richard
I also do not unerstand but ants of all things love high voltage electric contacts. I have seen them take up residence in both the well pressure switch and a distributor.