50 in the arrangement just described counter clockwise movement of the shaft will move the rotatable pivot arm 50 and bring with it the moveable end 42 of flexible closure diaphragm 40 and extend it across the opening 61 of the closure support 60 in some embodiments this motion alone will be sufficient for sealing and no camming is required but for the preferred operation especially under low pressure conditions rotation of shaft 70 will continue as the pivot arm 50 is stopped and ceases rotary movement by either being held against a stop as shown or by the resistance of the fully extended diaphragm closure 40 the continued motion will cause the substantially rounded surface 78 of the camming section 74 of shaft 70 to push against the front edge 54 of squared opening 53 and thereby radially move the closure support 60 so that the front surface 51 is pressed against the flexible diaphragm 40 and it into tightly sealed relation with the curved internal face 66 of the closure support 60 in this embodiment the camming arrangement of the shaft 70 and the closure support 50 function to move the front surface 51 both axially to position it and then radially to provide closing pressure rotation of the shaft 70 in the opposite direction likewise causes radial and axial movement but this time in reverse to open the valve to the position shown in fig3 b because this axial movement of the front edge 51 of pivot arm 50 can be likened to the movement of a brake shoe a the moveable front edge 51 having the capability for axial and radial movement can be referred to as a shoe forward nipple 67 on closure support 60 is shown to include flexible.