I replaced the ac/heater control switch with a newly purchased one on a 71 Cuda. I also rebuilt the box, cleaned all the lines, and function tested all the actuators/doors before installation using a Mity-Vac handheld unit. Everything is good there. I pulled the vacuum hose at the manifold and tried to pull a vacuum back to the switch and could not hold vacuum. I pulled the vac hose at the switch just passed the check valve and plugged the end, pulled vacuum again at the manifold end and held vacuum. I can pull the "cannon plug" and have "holding vacuum" on each port/line (I had performed this previously on the work bench to function test all box components/doors, etc.)

I found this issue chasing down an oscillating manifold vacuum reading when trying to tune my carb. Removed the AC/Heater manifold vacuum line, plugged off the manifold port and manifold vacuum improved. That's when I started chasing the vacuum leak down the ac/heater system.

So, my question is; based on what appears to be a vacuum leak within the switch body...this new switch is probably bad, right? Should it not hold vacuum internally? Anything else I might be overlooking here? Anybody got some trouble shooting techniques to share? It' been many months since I purchased the new switch, so I doubt I can get a swap out from vendor.....you know how it goes in that regard.

Your inputs are greatly appreciated.

Well the switch was my first thought. I remember redoing the switch in my 72 Dart decades ago and I used a thick silicone based grease to lube and seal the sliders inside the switch. I'd inspect for any obvious leaks in the switch first.

Here's a link to a rebuild article, one issue with it though, says to simply relubricate with Lubriplate, well Lubriplate is a brand name, not a specific lube. They probably mean white lithium.

https://www.forabodiesonly.com/mopar/ams/how-to-rebuild-a-vacuum-heater-control-switch.58/

Gotta make sure the "new" switch you got is correct for your application. They have been updated/superseded over the years, and the new ones, aren't same function as the older ones . . . . I went through that on my 70 Sport Satellite . . . . just a thought . . .

Cheers

Thanks for the comments. With these repops, does anyone have a good experience with a certain supplier, part #? Possibly, they are all made by the same manufacturer and distributed by various vendors.
When they function differently, I assume you mean that the select push buttons don't work the proper actuators (vacuum distribution).
Mine is more of a total failure, not holding a vacuum at all. I'll check the part number tonight. I ordered it from a major classic car parts supplier.
I'm not going to try and rebuild it. I'll just bite the bullet and order another replacement, but before I do so, I was seeking others input that the switch (body) should in fact hold vacuum constantly.
This is my first time dealing with this vacuum control switch. Thanks!

When I redid my 65's vacuum switch I had one stashed in a box I swapped in, no idea how or where I got it, but it worked fine Two differences, I could turn the AC on/off in defrost mode by pulling the button out after I put the stash switch in. and the electrical wiring in the back needed to be repinned, not the same. But once I sorted out what did what in each position I simply wired it that way, worked great to this day. Anyway it was PN 3895790, no idea if they are around NOS anymore, or even if an NOS one wouldn't fall apart after so long. Partsvoice shows no one stocking any, plenty of repop ones though, probably what you got and they all probably come from the same manufacturer, which means either they all suck or you got a defective one.

Did find an NOS one on feebay https://www.ebay.com/itm/NOS-389579...opar-A-B-E-Body-Challenger-/303390973356

Not sure anything I said will help, good luck