A valve assembly is disclosed for use in conjunction with a replaceable filter cartridge system. The valve assembly includes a head member having an interior valve chamber defining a central axis, and inlet and outlet passages extending parallel to and communicating with the interior valve chamber. The valve assembly further includes a valve member having an inlet path and an outlet path, adapted for rotation within the interior valve chamber of the head member between a filter position wherein the inlet path of the valve member is in communication with the inlet passage of the head member and a bypass position wherein the inlet path of the valve member is out of communication with the inlet passage of the head member. The valve assembly further includes a seal member seated in an upper surface of the valve member and positioned to seal against an opposing surface of the interior valve chamber. The seal member sealingly isolates untreated and treated fluid streams flowing through the inlet and outlet paths of the valve member, respectively, when the valve member is in the filter position, and sealingly isolates the inlet and outlet passages of the head member from the atmosphere when the valve member is in the bypass position to permit fluid to flow therebetween.