A modular pulmonary treatment system is provided and includes a patient interface device defined by a body which defines a hollow interior. The system also includes a first inhalation valve associated with the body and located along an inhalation flow path for providing selective fluid communication with the hollow interior of the body and at least one exhalation valve assembly detachable coupled to the body for discharging exhaled gas from the hollow interior to atmosphere. The at least one exhalation valve assembly comprises: (a) an exhalation valve cartridge including a housing having a perforated bottom wall with a post extending outwardly therefrom (b) a valve member configured to seat against the perforated bottom wall and including an opening through which the post extends and (c) a valve retainer for placement over the post, whereby the valve member is securely held in place against the perforated bottom wall.
