area of the recess 51 in the annulus gland 50 slight movement of the stem 40 is possible since the width of the recess 51 is larger than the diameter of the ring 41 since the stem 40 can be moved slightly the locking ring 75 can be exposed when the control piston 20 moves to a closed position fig4 illustrates the action of the valve assembly 10 in the event of a loss of pressure in the bladder element 80 as pressure is lost within the bladder element 80 for whatever reason the effect of the pressure of fluid in the bladder element 80 on the overpressure piston 30 and on the blocking locking stem 40 is reduced or eliminated thus the effect of the pressure of fluid in the annulus 2 on the control valve sliding piston 20 via the annulus port 63 forces the piston 20 toward a closed position as shown in fig4 the piston 20 has been moved back into a closed position exposing and freeing the expandable locking ring 75 the locking ring 75 has expanded to about the piston 20 preventing movement of the piston 20 toward the overpressure piston 30 thereby locking the valve assembly 10 in a closed position closing off ports 60 and 61 so that casing fluid no longer flows into the bladder element 80 the expansion of the locking ring 41 and its abutment against the wall of the recess 51 prevents further movement of the stem 40 toward the piston 20 the normal closed position of the valve assembly 10.