the internal floor 34 in combination defines a chamber of the valve well 30 the valve disk 40 is axially symmetrically positioned with respect to the valve well 30 and is kept in position by the pin 50 when a pressure p 1 which is greater than an opposing pressure in the chamber is exerted on the valve disk 40 the valve disk 40 is actively in a closed position in the closed position the valve disk 40 is seated on an annular rim 36 of the valve well 30 the valve disk 40 in the closed position forms a fluid seal with the annular rim 36 this prevents any fluid in the chamber of the valve well from escaping via spaces between the valve well 30 and the valve disk 40 the valve disk 40 is formed from a flexible resiliently deformable material and is of a larger diameter than the annular rim 36 when pressure p 1 is the same as the pressure in the chamber the valve disk 40 is passively in a closed position and seated on the annular rim 36 in operation when p 1 is lower than the opposing pressure in the chamber of the valve well 30 the valve disk 40 is actively in an open position the valve disk 40 flexes away from the annular rim 36 of the valve well 30 allowing fluid to escape via space between the valve disk 40 and the annular rim 36 of the valve well 30 in fig3 when pressure p 1 on the valve disk 40 is excessively high the valve disk 40 being made from a flexible resiliently.