side I believe that the port is a relief for the vapor canister to allow condensed gas vapors to return to tank (someone can correct me). You wouldn’t need/want strong vacuum for that. On the passenger side, appears to me additional vacuum is needed to operate the EGR valve, hence two places to draw from. But yes agree, not the simplest of engines to understand. [ http://www.twinturbo.net/nissan/300zx/forums/technical/view/1098236/NA-Vacuum-Diagram.html ]
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