taper the exact taper is not critical and tapers between 20 u00b0 and 120 u00b0 may be used the difference in taper between the seat and back up member may be from 0 25 u00b0 to 10 u00b0 the larger the difference and the larger the diameter of the depression 25 the more the axial end 19 will act as a lip seal against the valve seat the use of tapers over 20 u00b0 and the use of different tapers on the back up member and seat assure that the seal at the back up seal interface is self releasing and not self holding and that the seal contact will be at the axial end of the back up member to prevent extrusion of the ball between the seat and back up member a 46 u00b0 taper seat and 47 u00b0 48 u00b0 taper back up member is also useful preferably as illustrated in fig3 the dimensions of the ball with respect to the seat are such that sealing occurs along a cordal line not at the diameter of the ball this allows a good seal to be made if the ball becomes slightly compressed or obrounded due to the pressure of the spring 21 although i have herein shown a ball 20 as the primary seal member other shapes may be used such as for example a half ball seated in the depression 25 but of a larger full diameter than the back up axial.