acts on ball 44 in the position shown in fig1 a there is no pressure being applied and the biasing force of spring 42 keeps ball 44 against ball seat 40 taken as an assembly the components including ball seat 40 spring 42 and ball 44 comprise a check valve assembly when in the closed position as shown in fig1 a the passageway 12 is split into an upper segment which includes surface 46 on piston 48 and a lower segment which includes surface 50 on piston 48 other valve or restriction devices can be used without departing from the spirit of the invention such as a swing check valve an orifice or any other valve sensitive to pressure differential for its actuation or even less ideally an orifice piston 48 is illustrated in multi component form surface 46 is part of the piston housing 52 piston housing 52 is mounted adjacent upper housing 18 with seals 54 and 56 in between top sub 16 has a recess 58 a shear pin or shear screw 60 extends through a portion of piston housing 52 and into recess 58 as a result until the shear pin 60 breaks the position of the piston 48 is fixed with respect to.