Going by my experience of Nissan's pickups (Z-20, Z-22, & Z-24). They used a similar setup but it only let air in from the air cleaner into the intake. No exhaust connection. Did you mean to say intake instead of exhaust? But you are correct on the operation, spring pressure holds the valve shut and high vacuum during decel opens it. The easiest way to check it is to cap your vacuum fitting at the carb and plug the bypass air inlet from the air filter or pull the valve and plug the intake opening to see if it idles. Hope this helps!
