i think you may have problems upstream of the VCV? whats the measurement at the line going into the top of the VCV? i think it takes 20" and like you said, bleeds some off into the atmosphere. the VCV shouldn't output a full 20 to the trans.. i think it's something like 10 at idle down to 0 at wide throttle. mine isn't fully functioning so its like 5 or 6 at idle and 1 at WOT.. but even that results in smooth shifts. you can make adjustments by popping off the white dome on the front left of the VCV.. go slow with the adjustments because i guess it's really easy to break the thing !

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1981 NA 300D 310k miles