On my 81 w/300 six the purge vacuum is on the top port of the PVC.
Now I did not have any of the parts that would say when it was ok to purge for mine, like that 3 port valve that was shown.
So I hooked everything up like it should to the PVC with out the 3 port valve but then had a vacuum leak as it pulled to much vacuum for the canisters.
My fix was to install the smallest jet I had from my Holley box in the rubber line and this help control the "leak".
I was also getting poor MPG and thinking it was pulling fumes I cut & plugged the vacuum line for testing.
I dont think my MPG changed much so I hooked it back up a few weeks back but this time I took a plastic coupling melted the end closed and drilled a much smaller hole in the end so it would restrict the vacuum more and have less of a vacuum leak.
So far I do not get a gas smell in my garage other than from a bad fill hose I need to fix one day.
Dave ----