42 of the casing recesses 38 threaded on the outer end of stud 62 externally of the casing 12 is a cylindrical nut 68 threaded on the outer end of stud 64 externally of the casing 12 is a generally capped shape nut 70 these nuts are tightened firmly against the transverse walls 42 to firmly clamp these walls between the nuts and the bladder anchor 56 seal rings 72 are placed in the anchors and nuts as shown to seal the bladder securing means 52 54 to the casing 12 from the above description of the bladder 44 and the bladder securing means 52 54 and from the drawings it is evident that the ends of the bladder 44 are firmly but releasably secured to the casing end cap 16 and end wall 20 by virtue of the engagements of the bladder beads 48 50 in the anchor grooves 60 and casing recesses 38 respectively it is further evident that the bladder 44 is easily removable from and replaceable in the casing 12 thus removal of the bladder is accomplished by first removing the bladder securing nuts 68 70 which are accessible externally of the casing 12 for this purpose then removing the casing end cap 16 and finally withdrawing the bladder from the tubular casing portion 14 through its end opening 28 installation of the bladder in the casing.