In the vent-to-atmosphere CK*V 3-port version, the pilot spring chamber is vented externally to atmosphere through a vent hole in the plug. An o-ring is fitted over the hole to help prevent the ingress of dirt into the spring chamber. External leakage (“weepage”) equating to one drop for every 4000 cycles.This Atmospheric vented version cartridge fits into the 3-port cavity used by the standard cartridge. This feature can be useful, if the valve needs to be made insensitive to unanticipated system back pressure.NOTE: The vent port of the 3-port valve should never be plugged, as seal weepage will eventually cause valves to malfunction.