The little plastic vent dome on those caps is pressed on to a vacuum nipple, hook up a vacuum vent system as shown. Each piece has a function. Check valve keeps fuel fumes out of the distributor for obvious reasons. Filter ensures no dirt or contaminantes enter or restrict the orifice or check valve or find their way into the engine. The orifice has a small metered hole to restrict the flow and prevent the system from being too large of a vacuum leak. Small amount of air purging through the cap all the time makes a big difference in preventing ozone buildup and corrosion from the ozone buildup. Vans and SUVs already have one port of the PCV adapter taken for the HVAC system. The Dorman PCV cap has dual smaller fittings allowing for the distributor vacuum venting system to be added without affecting the use of the other port. Trucks with air injection will already have both ports and both in use.