I don't agree with the diagram. The arrow is pointing to a vent line with the other end hanging loose in the driver footwell. You'd hear some serious whistling under the dash if the vent line were sucking air to the tune of 11.4" Hg. You should test for vacuum at the line with the damper (green cylinder) that continues to the transmission modulator. IIRC the damper is directional. The other line should connect directly to the engine vacuum pump.

Sixto
87 300D