Positive Crankcase Ventilation is not simply a "smog device", it is a good thing for total engine health. Removing blow-by, condensation, and the like with a PCV system will result in a much healthier engine than any passive breather or road-draft tube will accomplish.

A proper "type IV" system will include two lines connecting to the crankcase. One will be to manifold vacuum, and will contain the PCV valve proper. The other will be the fresh air supply. Ideally, it will pull from inside the area contained by the air filter (to provide a filtered fresh air source), and will have a small dedicated screen-type filter.

During normal operation, a small amount of crankcase air (and any excess pressure) will be drawn through the PCV valve into the intake manifold to be burned in the engine. The air will be replaced by the fresh air side. During unusual periods of low vacuum and/or excessive blow-by, the fresh air line can handle the excess load, and flow "backwards" into the carb to be burned by the engine.