A two-stroke foam pump comprising: (a) a piston housing including a base wall and at least one sidewall extending from said base wall; (b) a piston assembly including: (i) a piston having a base end, said piston being selectively movable in said piston housing from a rest position wherein said base end lies proximate said base wall of said housing to a charged position wherein said base end lies farther away from said base wall of said housing, with movement of said base end away from said base wall serving to define a compressible mixing chamber that expands in volume as said base end is moved away from said base wall, and decreases in volume as said base end is moved toward said base wall, (ii) an outlet passage extending through said piston from said base end to an outlet, said outlet passage fluidly communicating with said compressible mixing chamber, (c) a liquid inlet in said housing communicating with said compressible mixing chamber; (d) a liquid inlet valve regulating fluid flow into said compressible mixing chamber through said liquid inlet, (e) an air inlet communicating with said compressible mixing chamber; wherein movement of said base end away from said base wall increases the volume of said compressible mixing chamber thus drawing air into said compressible mixing chamber through said air inlet and drawing liquid into said compressible mixing chamber through said liquid inlet, thereby creating a premix of liquid and air in said compressible mixing chamber, and wherein, thereafter, movement of said base end toward said base wall forces at least a portion of said premix of liquid and air through said outlet passage of said piston, characterized in that said piston housing includes a post extending from said piston housing, and said piston includes a bore including: a first section surrounding and engaging said post through a seal, a second section extending from said first section to said base end of said piston and having a diameter greater than that of said first section so as to surround said post and define an annular space between said post and said second section; a third section extending from said first section toward said outlet, wherein said piston moves relative to said post, and movement of said base end of said piston away from said base wall of said piston housing disengages the seal between said first section and said post when said second section reaches said seal. (a) a piston housing including a base wall and at least one sidewall extending from said base wall; (b) a piston assembly including: (i) a piston having a base end, said piston being selectively movable in said piston housing from a rest position wherein said base end lies proximate said base wall of said housing to a charged position wherein said base end lies farther away from said base wall of said housing, with movement of said base end away from said base wall serving to define a compressible mixing chamber that expands in volume as said base end is moved away from said base wall, and decreases in volume as said base end is moved toward said base wall, (ii) an outlet passage extending through said piston from said base end to an outlet, said outlet passage fluidly communicating with said compressible mixing chamber, (i) a piston having a base end, said piston being selectively movable in said piston housing from a rest position wherein said base end lies proximate said base wall of said housing to a charged position wherein said base end lies farther away from said base wall of said housing, with movement of said base end away from said base wall serving to define a compressible mixing chamber that expands in volume as said base end is moved away from said base wall, and decreases in volume as said base end is moved toward said base wall, (ii) an outlet passage extending through said piston from said base end to an outlet, said outlet passage fluidly communicating with said compressible mixing chamber, (c) a liquid inlet in said housing communicating with said compressible mixing chamber; (d) a liquid inlet valve regulating fluid flow into said compressible mixing chamber through said liquid inlet, (e) an air inlet communicating with said compressible mixing chamber; wherein movement of said base end away from said base wall increases the volume of said compressible mixing chamber thus drawing air into said compressible mixing chamber through said air inlet and drawing liquid into said compressible mixing chamber through said liquid inlet, thereby creating a premix of liquid and air in said compressible mixing chamber, and wherein, thereafter, movement of said base end toward said base wall forces at least a portion of said premix of liquid and air through said outlet passage of said piston, a first section surrounding and engaging said post through a seal, a second section extending from said first section to said base end of said piston and having a diameter greater than that of said first section so as to surround said post and define an annular space between said post and said second section; a third section extending from said first section toward said outlet,