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Throttle area has 4 kinds of connections;
Constant manifold pressure, after the throttle plate far enough to always have same pressure the cylinders actually see. Fuel pressure regulators use this, maybe dist too.
Manifold pressure off idle, ambient at idle, barely below the throttle plate, plate goes below it off idle. Maybe dist, used to control other valves.
Manifold pressure at idle, ambient off idle, barely above the throttle plate (other side from above), plate goes above it off idle. Mostly used to control other valves.
Constant ambient, below the throttle plate far enough to never see manifold pressure except at maybe full throttle and high rpm, then its still ambient less pressure drop of air filter etc. Breather lines for air that has already gone through the air meter.
The 928 had different types of distributor over the years, some are vacuum advance, some are vacuum retard, or something like that, some maybe both. You need to find the vacuum diagram for YOUR car and fix all the stuff that is routed wrong.
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