My Formula doesn't have a visible switch on top of the canister either. It's hooked up in a similar fashion, a vacuum line and 3/8 hose to the carb. There is no purge valve on the intake of that one either.
As far as I know, both systems on my Chevelle and Firebird run all the time, they've never created too much vacuum at the tank or created any kind of issue. In fact without the tank venting the opposite problem occurs. Remember the vacuum connection is switched (ported) at the carb so it's not active at idle anyway. Matter of fact, I would think you'd want it functioning most all the time anyway, because a tank that isn't vented at all causes way worse issues than a charcoal canister ever will. I've had that issue before and fuel delivery becomes a nightmare.
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2019 Pontiac Heaven class winner
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