Possibly a bit but no more than a purge line from a charcoal canister would; you don't have a constant flow of vapor anyways it all pretty much gets 'consumed' within the first few seconds of startup and really just builds up once the car is shut off. It was kind of a stop-gap measure to keep gas vapors from venting to the open air after shutoff, before they started using charcoal canisters.
I finally found this article written by our local 'wizard' Dan Stern (slantsixdan), explains everything you need to know about the "ECS" system on our '70-71 cars:
Fixing 1970s Evaporation Control Systems
