A diaphragm valve assembly 10 for a respiratory system is provided, comprising a valve housing 12 having a first port 16 arranged for fluid communication with a fluid supply and an outlet 30 spaced from the first port by a valve chamber 68. A diaphragm member 50 selectively controls the flow between the first port and the outlet. The valve housing has an opening 34 across which the diaphragm extends. A lid 14 cooperates with the valve housing to trap the diaphragm member at 50A, forming a seal, and defines and isolates a control chamber 70 between the diaphragm member and the lid. Pin 60 in recess 48 aids in aligning the lid and valve. The lid may have a control port connector formation integrally formed therein. In use, high pressure in the conduit 32 (e.g. from exhalation) lifts the valve sealing flange 52 and support flange 54 off the valve seat 42 and the fluid can exit through the valve chamber 68 to outlet 30. When the pressure in the conduit 32 is lower than the pressure in the control chamber 70, the diaphragm deforms and sealing flange 52 contacts the valve seat 42, closing the valve.
