It's a very simple system with just 3 mechanical vacuum switches which operate pneumatic servos- no electronics or anything. You probably just have a vacuum leak or disconnected line somewhere in the system, which you can trace via a handheld vacuum pump.
A cheap temporary fix if you find that one of your pneumatic servos leaks is to just cap off the line to it, and then everything else will start working again.
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'84 760 D24T/M46
'86 Isuzu Trooper Turbo Diesel
'01 VW Golf TDI
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