Maybe a clarification would help based on the '66 example. You need to have an in portal, and an out portal to have cross ventilation. Sorta like trying to cool off your house on a summer's night. You won't get much if you open two windows next to each other. The ideal is at opposite sides of the engine (side to side, front to back, whatever) Typically the hose leading from a vent, be it a cap on the valve cover or the fill pipe itself, is the intake supply that connects to the air filter for filtered air. See this for concept; http://www.clubhotrod.com/forums/att...&postid=100112

To go along with Steve and Mike, those of us who are old enough to have worked on pre-PCV engines remember all the sludge build up we'd see on rocker assemblies in particular. These were on engines that had no ventilation at the valve covers. Moisture and other by-products of combustion joined to form acids that led to the formation, along with particulates, of sludge. Modern oil formulations do a better job of inhibiting sludge formation, along with unleaded gasoline, but the PCV and concurrent venting of the valve cover area, have a lot to do with it.