Patent ID: 7353746

Claim:
A two-stage pump comprising: (a) a first chamber defined by a first chamber wall; (b) a first piston which is movably located in said first chamber; (c) a plunger which is attached to said first piston; (d) a second chamber defined by a second chamber wall; (e) a second piston which is movably located in said second chamber; (f) a locking mechanism which causes activation of said plunger to move said first piston in said first chamber without moving said second piston in said second chamber when said plunger is rotated in a first direction to a first position, and causes activation of said plunger to move said second piston in said second chamber without moving said first piston in said first chamber when said plunger is rotated in a second direction to a second position; wherein said second piston comprises: [i] an annular pressure ring having an outer diameter which is slightly less than an inner diameter of said first chamber; [ii] an annular relief ring which is spaced apart from said pressure ring and has an outer diameter substantially equal to the outer diameter of said pressure ring; [iii] said relief ring having a plurality of discontinuities located about its periphery; [iv] an O-ring which fits between said pressure and relief rings to seal said first piston in said first cylinder; [v] aid O-ring, said pressure ring, said relief ring and said discontinuities being arranged such that when said first piston is moved into said first chamber said O-ring forms a seal between said pressure ring and said first chamber wall, and when said first piston is moved out of said first chamber said O-ring collapses into said discontinuities such that said O-ring does not form a seal between said relief ring and said first chamber wall.