referring to fig1 the assembly of the present invention is indicated generally at 10 in position within a production tubing string t . the assembly 10 includes a packer p having a sealing means s and anchoring means a . a valve mechanism 11 operated by a control mechanism 12 is secured to the packer p by a connecting sleeve 14 . in the illustration of fig1 the packer anchoring means a are illustrated as expansible dogs which have been radially expanded into the annular collar recess formed between upper and lower tubing sections t - 1 and t - 2 , respectively which are joined together by a conventional collar c . an elastomeric material is employed for the sealing means s so that upon expansion of the sealing means , a pressure seal is formed between the body of the packer p and the surrounding conduit t . the packer p is of the type which may be set within a tubing string and subsequently retrieved in conventional manner . the packer preferably includes a large central opening to permit relatively unimpeded flow of fluids through the tubing . where collar recesses are not available , or where otherwise desired , the anchoring dogs a may be replaced by conventional slips having teeth formed along their external surfaces and adapted to be moved radially outwardly into friction locking engagement with the internal walls of the tubing string . the packer p may be similar to the packer described in u . s . patent application ser . no . 857 , 047 filed sept . 11 , 1969 entitled anchor assembly for well tools such as packers and the like , or u . s . patent application ser . no . 272 filed jan . 2 , 1970 entitled tool string assembly for use in wells . as may best be seen by reference to fig2 the valve mechanism 11 includes a tubular housing section 11a which surrounds flapper type valve closure member 11b adapted to be pivoted about a mounting pin 11c . when in closed position , as illustrated in fig3 the valve closure member 11b engages and seals with an annular seat 11d formed about a flow passage 11e which extends through the valve housing 11a . when the assembly 10 is anchored in position within a well bore with the seal s expanded to provide the desired pressure seal , well effluents are forced to flow through the flow passage 11e and up through the central opening in the packer p . referring jointly to fig2 and 8 , it may be seen that the control mechanism 12 is received in a semicircular recess 11f formed in the external body of the valve housing 11a . as illustrated in fig7 and 8 , the composite cross - sectional area of the assembly is substantially circular in shape . the control mechanism 12 is threadably secured to the valve housing 11a as illustrated at 11g in fig2 and is supported in the recess 11f by support bolts r illustrated in fig8 . proper spacing of the control 12 with respect to the flapper valve member 11b is effected by changing the threaded engagement 11g between the control and the housing 11a . once properly adjusted , the control is held in position by the bolts r . linear movement exerted by components contained within the control mechanism 12 is conveyed to the flapper closure element 11b through a shaft extending from a connecting linkage 12a which projects from the control mechanism into the valve mechanism . the linkage 12a includes a flexible shaft portion at its upper end and an enlarged tubular body section at its lower end . fig7 of the drawings illustrates the separable connection formed between the upper end of the linkage 12a and the closure element 11b which permits separation of the two structures when the control mechanism 12 is removed from the valve mechanism 11 . the upper tip of the linkage 12a is bent at a right angle to the main shaft and is loosely received in a lateral bore formed in the flapper element 11b . as the valve element rotates about the mounting pin 11c , the receiving bore moves in an arc and rotates about the tip of the linkage 12a . it will be appreciated that movement of the linkage 12a toward the pin 11c pivots the valve element 11b toward its closed position while movement away from the pin pivots the element to its open position . as best illustrated in fig3 the pivotal movement of the flapper closure element 11b is accommodated in the linearly moving linkage 12a by flexing of the linkage &# 39 ; s main shaft portion . details in the preferred form of the control mechanism 12 may best be described with reference to fig4 where it is seen that a plural part , tubular housing 12b encases the components of the control 12 . internal components of the control include upper and lower bellows sections 12c and 12d , respectively . the bellows sections are of a conventional type adapted to axially elongate or contract in response to pressure differentials existing across the bellows wall . the upper bellows section is secured to the housing at 12e and the lower bellows section is secured to the housing at 12f . the free end of the bellows section 12c is connected to a central driving rod 12g while the free end of bellows section 12d is connected to a cap 12h equipped with a removable plug 12i . the lower end of housing 12b is equipped with a removable cap 12j which is threadedly engaged to the control housing . annular o - ring seals 12k are positioned between the cap 12j and housing 12b to provide a pressure tight seal . a conventional valve 12j &# 39 ; is positioned internally of the housing to provide a means for inserting gas into a pressure tight dome or chamber 12m formed between the bellows section 12d and the internal walls of the control housing 12b . an internal shoulder provided by a component part 12b &# 39 ; of the control housing 12b supports the lower end of a coil spring 12n . a portion of the spring 12n extends upwardly within a depending sleeve carried by an axially movable valving component 12p and the uppermost end of the spring engages and bears against the body of the valving component 12p to bias the component upwardly . an annular , resilient sealing member 12p &# 39 ; is carried on the valving component 12p . in the lowermost axial position of component 12p illustrated in fig4 the seal 12p &# 39 ; engages and seals with an internal seating surface 12b &# 34 ; to isolate an enclosed area 12d &# 39 ; below the seal from an enclosed area 12c &# 39 ; above the seal . the upper end of valving component 12p includes a sleeve which extends over the lower end of driving rod 12g . upward movement of component 12p is transmitted to the drive rod 12g by engagement of shoulders formed on each of the two components . the component 12p engages the drive rod 12g in a free sliding fit whereby the valving component 12p and rod 12g may move in opposite directions under certain circumstances to be hereafter described . drive rod 12g is a plural part component which includes a shoulder section 12g &# 39 ; projecting radially away from the main body of the drive rod . a coil spring 12q is confined between the shoulder section 12g &# 39 ; and an internal shoulder formed within the housing 12b . a second coil spring 12r is confined between the shoulder section 12g &# 39 ; and a shoulder 12a &# 39 ; formed on the movable linkage 12a . a second shoulder 12a &# 34 ; formed on the linkage 12a supports the base of a coil spring 12s and the upper end of spring 12s engages an annular , radially extending shoulder 12g &# 34 ;. a triggering member 12t having an annular recess 12t &# 39 ; formed about its central portion is supported on top of the drive rod 12g . between the upper end of member 12t and the linkage 12a is confined a power spring 12u . a plurality of small metal balls 12v are carried in radial bores 12w extending through the tubular body section of linkage 12a . as will be seen , the balls 12v are retained in a radially extended position when illustrated trigger component 12t is in the position illustrared in fig4 . when in a radially extended position , the balls 12v engage upper and lower shoulders formed in the internal surfaces of the housing 12b as illustrated in fig4 to prevent axial movement of the linkage 12a . when the triggering component 12t moves axially upwardly so that the recess 12t &# 39 ; is coincident with the balls 12v , the balls are permitted to retract radially inwardly and axial movement of the linkage 12a is then permitted . in employing the self - contained , retrievable valving assembly of the present invention , the assembly is initially lowered into the well conduit t by means of a suitable wireline string or other apparatus until the desired subsurface location is reached . the slips or anchoring means a and the seal s are thereafter set and the positioning means is removed from the tubing string . prior to positioning the assembly within the string t , the chamber 12m is filled with a compressible gas at a desired pressure . the gas pressure in the chamber is determined by the well conditions which are to be encountered by the assembly . during the lowering procedure , the valve closure member 11b is retained in an open position by a hold open mechanism indicated generally at 13 in fig5 . the mechanism 13 includes a central bore 13a extending axially through the mechanism to permit flow through the assembly as the assembly is being lowered into a fluid containing tubing string . during the downward movement of the member 13 through the tubing string , the dogs 13b are depressed radially inwardly and spring out when they are coincident with the recess 14a . the lower movement of member 13 is limited by its engagement with a shoulder 14b formed internally of the sleeve 14 . the lower end of member 13 includes a downwardly extending projection 13c which prevents the flapper valve 11b from moving into closed position . once the assembly has been properly positioned within the tubing string t , the member 13 may be removed by a suitable retrieving means of conventional construction . removal of the member 13 is effected with back pressure existing at the well head so that the flapper valve 11b remains in its open position even with the member 13 removed . the mechanism 13 may also be employed to remove the self - contained assembly 10 from the tubing string t . when the assembly 10 is in position within the tubing string t , well pressures above a predetermined minimum valve act against the external surface of bellows section 12c tending to compress or axially foreshorten the bellows which in turn retains the drive rod 12g in its lowermost axial position . in the event unusually high pressures in the tubing string t are encountered , damage to bellows sections 12c and 12d is prevented by action of the sealing component 12b &# 39 ; which prevents the pressure acting against the bellows 12c from being communicated to the internal bellows area 12d &# 39 ;. since the internal bellows area 12c &# 39 ; and 12d &# 39 ; and the area confined between the two sections are completely filled with an incompressible fluid , foreshortening of the bellows section 12c is limited to prevent the bellows from collapsing . in the reverse situation where an unusually low pressure acts against the external surface of bellows 12c , the pressure charge in the gas chamber 12m tends to compress and axially foreshorten the bellows 12d which in turn causes the valving component 12 p to move to its axially upper position where the sealing means 12p &# 39 ; is engaged with the seat 12b &# 39 ;&# 34 ;. the fluid confined internally of the bellows chamber area 12d &# 39 ; prevents the bellows from collapsing and the valving action of component 12p prevents the pressure in chamber 12d &# 39 ; from being communicated to the bellows chamber 12c &# 39 ; so that the bellows section 12c is not distorted in an outward direction . fig4 illustrates the control mechanism as it appears under normal operating conditions with the valve open . when the pressure of the tubing string drops below a predetermined minimum value , the gas charge in chamber 12m exerts a compressing force on the fluid contained in chamber 12d &# 39 ; which is transmitted to the fluid in chamber 12c &# 39 ;. the bellows section 12d thus compresses and bellows section 12c expands . during this process , the seal 12p is moved upwardly and when it moves sufficiently upwardly , it engages and seals against the internal seating surface 12b &# 39 ;&# 34 ;, to provide bellows protection . the spring 12n forces the valving component 12p to follow the upward movement of drive rod 12g . because of the slip joint connection between the drive rod 12g and component 12p , the biasing force of spring 12n is required to cause the element 12p to follow the movement of rod 12g . the slip joint connection between rod 12g and element 12p is required because the fluid contained within the chamber sections is not absolutely incompressible and high pressure acting externally of the bellows 12c may tend to push the drive rod 12g slightly upwardly while the valving component 12p is held tightly in sealing position against seat 12b &# 39 ;&# 34 ;. the slip joint permits this small relative movement to take place . during the upward movement of the drive rod 12g , the springs 12r and 12u are being compressed . when the recess 12t &# 39 ; on member 12t moves up to the balls 12v , the balls can back into the recess and the linking assembly 12a is freed from locking engagement with the housing 12b . the compressed force of spring 12r is then suddenly released to push the linkage 12a quickly upwardly causing the flapper valve 11b to snap shut . the spring 12u also reacts against the top of the element 12t to assist in the snapping action .. during closure of the valve and movement of the linkage 12a upwardly , the spring 12s is compressed by the movement of the linkage shoulder 12a &# 34 ; toward rod shoulder 12g &# 34 ;. spring 12s stays highly compressed as long as the valve is closed . when pressure across the closed valve is equalized by pressuring up at the surface , the resilient force of spring 12s tends to drive the linking assembly 12a back downwardly and the rod 12g upwardly . this pulls the linking assembly 12a downwardly through the housing 12b and the spring 12u acts downwardly on the member 12t so that the balls 12v are pushed out of the recess 12t &# 39 ; and into the bore 12w with the member 12t then moving further downwardly under the influence of spring 12u until the valve is again in the position illustrated in fig4 . a modified control mechanism indicated generally at 112 is illustrated in fig6 a and 6b . the modified control mechanism 112 includes a spring mounted , flexible linkage 112a carried within a spring mount 112a &# 39 ;. the spring mount 112a &# 39 ; is carried in a recess formed at the upper end of a drive rod 112g . the lower end of the drive rod is telescoped over a valving component 112p equipped with an annular resilient sealing means 112p &# 39 ;. the sealing element 112p &# 39 ; is designed to move between lower and upper sealing surfaces 112p &# 34 ; and 112p &# 34 ;&# 39 ;, respectively formed within the internal walls of a tubular control housing 112b . a coil spring 112n extends between the lower end of the valving component 112p and an internal shoulder formed within the control housing 112b to cause the component 112p to follow upward movement of the rod 112g . the lower end of the control 112 , as illustrated in fig6 b , includes a removable cap 112j , an annular sealing o - ring 112k and a conventional gas valve 112j &# 39 ;. with the cap 112j removed , gas is vented through the valve 112j &# 39 ; into a pressure dome 112m contained within the tubular control housing 112b . the bellows portion of the control mechanism 112 includes a single bellows section 112c having its lower end 112e secured to the control housing 112b and its upper end 112e &# 39 ; secured to the drive rod 112g . a piston / 12h equipped with an annular o - ring seal 112d is adapted to move axially through a receiving piston bore in the housing 112b to provide substantially the same function as the bellows 12d employed in the preferred form of the control shown in fig4 . the enclosed areas 112d &# 39 ; and 112c &# 39 ; are filled with a non - compressible fluid to provide protection for the bellows 112c in a manner similar to that already described with reference to fig4 . the linking means 112a is designed to transmit the linear motion induced by expansion and contraction of the bellows 112c and travel of the piston 112h to a flapper valve element 111b . the resilient mounting of the linkage 112a in the spring 112a &# 39 ; permits the linkage to flex and bend at its upper end to follow the pivotal movement of the flapper valve 111b during its opening and closing motion . the foregoing disclosure and description of the invention is illustrative and explanatory thereof , and various changes in the size , shape and materials as well as in the details of the illustrated construction may be made within the scope of the appended claims without departing from the spirit of the invention .