Let me add another view. Below is a cutaway showing the diaphragm and the vacuum channel leading to it (sort of squiggly red line). One end of that line leads to your intake manifold and the other is below the diaphragm (with a tiny vacuum air bleed line terminating in the bore of the carburetor. As long as no gas enters the recessed area below the diaphragm, all is okay. Vacuum pulls against the bottom of the diaphragm and this in turn pulls it down against the strength of the lower diaphragm spring. Once there is not enough vacuum to overcome the strength of the spring, you'll start running richer as the metering rod will travel upward placing the smallest portion of the rod in the main jet...which feeds more fuel up through the passage leading to the nozzle and making the mixture richer.
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
