'88 GMC/Chevy heater control heads

My wife's 1988 S-15 Jimmy has a vacuum leak in the heater/air conditioner control head (behind the levers which control temperature and air flow). Two competent shops AND a not-quite-so-competent dealer have been unable to
fix this, because all of the replacement parts have been defective, right out of the box. Can someone suggest where to find a control head which doesn't leak, how to fix a leaking one, or how to find someone (in Colorado or perhaps Nebraska) who truly knows how to fix one?
These control heads are a wonder of vacuum engineering...when they work. Two plastic discs rotate against each other, directing vacuum through an intricate maze of passages, thus moving vacuum-powered actuators which control the air flow. In many cases, one of the plastic discs seems to be ever so slightly warped, thus "letting the vacuum out." :-)
Paul
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Paul Goble wrote:

I too have the same problem, Can hear bad leaking behind dash, depending on my speed and where i position the levers changes the pitch and amount of vacuum leak I can hear. Just out of curiosity, I know a vacuum leak in the engine bay can cause poor performance. Would this leak effect my trucks performance?
Dustin
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<<Jimmy repair.txt>>

...
It could. If a leak is small enough, the vacuum pump can maintain vacuum in the rest of the system. But it doesn't take much of a leak to impair the whole, interconnected vacuum system. In an earlier episode of our vacuum leak saga, repairing a vacuum leak in the heater controls also fixed problems with rough idling, stalling, and poor performance.
I do have a bit of an update: we took the Jimmy to an air conditioning specialist shop. Just in the process of diagnosis, they reinstalled the control head and fixed 60% of the problem. That's pretty clear proof that in addition to defective parts, both earlier shops AND the dealer were underskilled, to put it nicely. They say that the rest of the problem is a combination of defective parts and debris from botched repair attempts. That's pretty clear proof that in addition to defective parts, both earlier shops AND the dealer were underskilled, to put it nicely. Now the bad news: the specialty shop said they needed to order a tool before they could complete the repair, and that they'd call us when it came in. That was a month ago. I suspect the job is difficult enough that they just don't want the business--or the future business that we planned to bring them if they did a good job. :-(
Still looking for a competent mechanic,
Paul in Colorado Springs
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