extend radially from the rod portion 72 into close proximity with the walls of the bore 36 for engagement with the latter to guide the lower portion of the actuator rod 72 in this manner spaced portions of the valve actuator 64 are guided by the head portion at the upper end and by the fin portions 76 at the lower end so that during opening and closing movement of the valve element 70 on the annular seat 48 the rod like portion 64 is maintained in axial alignment with the bore 36 of the conduit 26 this insures good seating of the valve element 70 and valve seat or lip 48 such guiding and alignment also insures an axial location of the spherical end portion 66 relative to the conduit 26 a modified form of guiding arrangement is shown in fig5 in which the valve seat member 40 is provided with an annular tube like skirt 77 instead of the guide element 50 shown in fig1 and 3 in this arrangement the head 66 of the actuator is provided with a plurality of protrusions 78 which have semi spherical outer surfaces the protrusions 78 are engageable with the inner walls of the tubular skirt 77 to maintain the actuator head 66 in axial alignment with the annular lip 48 forming the valve seat as best seen in fig1 conduit member 26 is provided with a vent arrangement formed by radially extending holes 79 which communicate the.