Abstract: A valve for controlling flow of fluid through an aperture in a barrier comprises a valve body including a pin and having a continuous sealing surface surrounding at least one opening through the valve body, and a diaphragm of resilient material. The diaphragm has an opening surrounded by an inner margin and is disposed with the pin extending through the opening and with a main surface presented towards the continuous sealing surface of the valve body. The inner margin of the diaphragm is in a state of tension around the pin of the valve body, whereby the diaphragm is prestressed to a condition in which the main surface of the diaphragm engages the sealing surface of the valve body.