A multi-channel positive displacement piston pump apparatus comprising: a motor; and a plurality of positive displacement piston pumps (12a, 12b, 12c, 12d) driven by said motor, said plurality of pumps (12a, 12b, 12c, 12d) being aligned in a stacking direction, characterized in that each of said plurality of pumps (12a, 12b, 12c, 12d) comprises: a pump housing (34, 101) defining a central longitudinal bore (44, 44'); and a pump piston axially and rotatably slidable within said central longitudinal bore (44, 44') for pumping a liquid through said pump housing (34, 101), wherein said pump housing (34, 101) further comprises: an inlet port; an outlet port, said outlet port facing in a same direction as the inlet port generally perpendicular to said stacking direction; a first transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said inlet port to said central longitudinal bore (44, 44'); a second transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said central longitudinal bore (44, 44') to said outlet port; a longitudinal groove (60, 60') extending between said first transverse bore and said second transverse bore for conveying the liquid therebetween; and an annular groove (62, 62') formed in said central longitudinal bore (44, 44') at a juncture of said central longitudinal bore (44, 44') and said second transverse bore for conveying the liquid from the longitudinal groove (60, 60') around the piston to the second transverse bore. a motor; and a plurality of positive displacement piston pumps (12a, 12b, 12c, 12d) driven by said motor, said plurality of pumps (12a, 12b, 12c, 12d) being aligned in a stacking direction, characterized in that each of said plurality of pumps (12a, 12b, 12c, 12d) comprises: a pump housing (34, 101) defining a central longitudinal bore (44, 44'); and a pump piston axially and rotatably slidable within said central longitudinal bore (44, 44') for pumping a liquid through said pump housing (34, 101), wherein said pump housing (34, 101) further comprises: an inlet port; an outlet port, said outlet port facing in a same direction as the inlet port generally perpendicular to said stacking direction; a first transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said inlet port to said central longitudinal bore (44, 44'); a second transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said central longitudinal bore (44, 44') to said outlet port; a longitudinal groove (60, 60') extending between said first transverse bore and said second transverse bore for conveying the liquid therebetween; and an annular groove (62, 62') formed in said central longitudinal bore (44, 44') at a juncture of said central longitudinal bore (44, 44') and said second transverse bore for conveying the liquid from the longitudinal groove (60, 60') around the piston to the second transverse bore. a pump housing (34, 101) defining a central longitudinal bore (44, 44'); and a pump piston axially and rotatably slidable within said central longitudinal bore (44, 44') for pumping a liquid through said pump housing (34, 101), wherein said pump housing (34, 101) further comprises: an inlet port; an outlet port, said outlet port facing in a same direction as the inlet port generally perpendicular to said stacking direction; a first transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said inlet port to said central longitudinal bore (44, 44'); a second transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said central longitudinal bore (44, 44') to said outlet port; a longitudinal groove (60, 60') extending between said first transverse bore and said second transverse bore for conveying the liquid therebetween; and an annular groove (62, 62') formed in said central longitudinal bore (44, 44') at a juncture of said central longitudinal bore (44, 44') and said second transverse bore for conveying the liquid from the longitudinal groove (60, 60') around the piston to the second transverse bore. an inlet port; an outlet port, said outlet port facing in a same direction as the inlet port generally perpendicular to said stacking direction; a first transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said inlet port to said central longitudinal bore (44, 44'); a second transverse bore communicating with said central longitudinal bore (44, 44') for conveying the liquid from said central longitudinal bore (44, 44') to said outlet port; a longitudinal groove (60, 60') extending between said first transverse bore and said second transverse bore for conveying the liquid therebetween; and an annular groove (62, 62') formed in said central longitudinal bore (44, 44') at a juncture of said central longitudinal bore (44, 44') and said second transverse bore for conveying the liquid from the longitudinal groove (60, 60') around the piston to the second transverse bore.