Oh here, I'm a sucker.
http://www.continentalimports.com/ser_ic20242.html
Quote:
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All testing and repairs should be done after viewing the pertinent vacuum schematic, as the system is intertwined with supply of EGR and intake pressure control systems. The system has various other controls depending on installation. On most models, the vacuum is not tied to the proportioning valve until some throttle is taken up, closing a vacuum microswitch on the valve cover. This keeps the vacuum high at idle which keeps some models in second gear until throttle take-up. It also reduces the closed throttle downshift to an almost undetectable level.
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If you put a vac gauge on a properly working setup in the line to the modulator, and slowly open the throttle, you'll see what he's talking about.
As I said, you don't have to have it, but it's there for a reason, and can become crucial to get a satisfactory shift on some well worn examples.
Most people don't see the relationship, and I've had this conversation more than once. I did finally contact Mr. Brotherton on this matter and he confirmed I was not nuts.
Enjoy..