A pump comprising: a pump body defining a fluid chamber , a seat disposed at an end of the fluid chamber , and inlet and outlet ports in fluid communication with the fluid chamber ; an inlet check valve directly connected to the inlet port and to permit fluid flow into the inlet port ; an outlet check valve directly connected to the outlet port and to permit fluid flow out of the outlet port ; a piston housing coupled to the pump body , the piston housing defining a piston chamber ; a piston assembly disposed at least partially within the piston chamber and at least partially within the fluid chamber , the piston assembly including a piston and a poppet connected to the piston in proximity to the pump body , the piston including a flange at an end of the piston opposite the poppet ; and a gland disposed between a portion of the piston housing and the pump body , an actuating volume of the piston chamber being defined between the flange of the piston and the gland ; wherein the poppet has a poppet head , a diaphragm portion , and a poppet flange ; the poppet head being axially connected to the diaphragm portion coupled to the poppet flange ; the poppet head being connected to the piston ; the poppet flange being disposed axially between the gland and the pump body ; and the diaphragm portion being to rollingly engage the gland in response to movement of the piston ; characterized in that the poppet head is adapted to extend to contact the seat of the pump body and is coextensive with the seat of the pump body, wherein the seat of the pump body is configured such that the poppet head can contact the seat of the pump body at a point along the longitudinal axis of the piston . a pump body defining a fluid chamber , a seat disposed at an end of the fluid chamber , and inlet and outlet ports in fluid communication with the fluid chamber ; an inlet check valve directly connected to the inlet port and to permit fluid flow into the inlet port ; an outlet check valve directly connected to the outlet port and to permit fluid flow out of the outlet port ; a piston housing coupled to the pump body , the piston housing defining a piston chamber ; a piston assembly disposed at least partially within the piston chamber and at least partially within the fluid chamber , the piston assembly including a piston and a poppet connected to the piston in proximity to the pump body , the piston including a flange at an end of the piston opposite the poppet ; and a gland disposed between a portion of the piston housing and the pump body , an actuating volume of the piston chamber being defined between the flange of the piston and the gland ; wherein the poppet has a poppet head , a diaphragm portion , and a poppet flange ; the poppet head being axially connected to the diaphragm portion coupled to the poppet flange ; the poppet head being connected to the piston ; the poppet flange being disposed axially between the gland and the pump body ; and the diaphragm portion being to rollingly engage the gland in response to movement of the piston ; characterized in that the poppet head is adapted to extend to contact the seat of the pump body and is coextensive with the seat of the pump body, wherein the seat of the pump body is configured such that the poppet head can contact the seat of the pump body at a point along the longitudinal axis of the piston . <EOS>