I'll need to look into how the Pinto allows air into the tank to avoid collapsing the tank, but in the case of my CIS K jetronic GTI, the tank has to have a non vented cap in order to hold pressure to prime the electric fuel pump. Later , the Golf used a in tank pump and the External pump to send fuel to the fuel injection distributor. This was to avoid vapor lock in the Lines and the Evap was changed to have electrically controlled purge valves in the system to avoid the pre OBD 1 systems showing a pressure drop in the evap if the cap was left loose, or missing.
In summary, There must be a way for fresh air to enter, but not leave the tank ( Except as fumes) so the canister system can do it's job. If it isn't hooked up correctly, you have potential for an air leak while running if the Purge valve hangs up. The mixture settings of the Carb are recalibrated to allow for the intake of addition fuel vapor from the canister. To unhook or incorrectly connect the lines is the way to frustration or Operation that is Pre Smog control in nature.
It's just me venting Again
Pintosopher , a Burp and emitted gas fume from enlightenment