is preferably formed of an elastomeric resin for low sliding friction such as delron the purpose of the guide ring 78 is to enable guided movement of the movable outer member 48 with respect to the stationary inner member 50 and between the two extreme positions shown in fig3 and 4 in fig3 it may be seen that the movable outer member 48 is substantially in the lower most down position where a ledge 84 butts against a threaded stop 86 in contrast in fig4 the movable outer member 48 has been actuated and slidable moved upwardly under the guidable engagement of the guide ring 78 until a ledge 88 engages the threaded stop 86 within a chamber 90 defined between the stationary inner member end wall 60 and the movable outer member end wall 74 there is provided a sealed preformed bladder 92 formed of two pieces joined by heat sealing at a joint 94 bladder top piece 92a is preformed to the shape of dome shaped top end wall 74 and bladder bottom piece 92b is preformed to the shape of the step shaped bottom end wall 60 the bladder is formed of polyurethane with a cloth backing layer 95 for lower temperature usage the cloth backing layer 95 is not required the cloth backing layer 95 surrounds the entire bladder 92 and serves to reinforce the bladder material for use at high operating temperatures at extreme.