Patent ID: 8347954

Claim:
A plunger having an upper end and a lower end, and defining an internal chamber, for intermittently lifting fluids from a well having a bottom well stop means, said plunger comprising: (a) at least one hollow cylindrical seal mandrel disposed between the upper end and lower end, wherein the seal mandrel defines a plurality of openings; (b) a resilient seal sleeve attached to the seal mandrel in a fluid-tight manner, covering the seal mandrel openings; (c) a valve assembly disposed at one end of the plunger, comprising a valve body defining a valve opening, a valve stem wherein a valve is slidingly disposed within the valve body, and is moveable between a first position wherein the valve opening is closed and a second position wherein the valve opening is open; and (d) an upper ball detent system which cooperates with an upper detent profile formed on the valve stem, and a lower ball detent system which cooperates with a lower detent profile formed on the valve stem, wherein each ball detent system comprises a ball biased in an inward radial direction by a spring, said ball cooperating with the corresponding detent profile to resist vertical movement of the valve stem wherein the force required to overcome the upper ball detent system, thereby closing the valve, is less than the force required to overcome the lower ball detent system, thereby opening the valve.