A pump (20, 20b) comprising: a reciprocating piston (10a, 10b) disposed in a pump housing (21, 21b), the housing (21, 21b) comprising an inlet (35, 35b), an outlet (36, 36b), an interior and a middle seal, the piston (10a, 10b) comprising a proximal section (28, 28b) connected to a pump section (29, 29b) at a transition section (31, 31'), the proximal section (28, 28b) being linked to a motor, the proximal section (28, 28b) having a first maximum outer diameter, the pump section having a second maximum outer diameter that is greater than the first maximum outer diameter, the pump section (29, 29b) comprising a distal recessed section (13a, 13b), the pump section (29, 29b) extending between the transition section (31, 31') and a distal end (33, 33b), the pump section (29, 29b) of the piston (10a, 10b) being at least partially and frictionally received in the middle seal of the housing (21, 21b), the housing (21, 21b) and piston (10a, 10b) defining two pump chambers including a first chamber (42, 42b) defined by the distal recessed section (13a, 13b) and distal end (33a, 33b)of the pump section of the piston and the housing (21, 21b), and a second chamber (44, 44b) defined by the transition section (31, 31') and proximal section (28, 28b) of the piston and the housing (21, 21b), wherein the first and second pump chambers (42, 42b; 44, 44b) are axially isolated from each other by the middle seal and the pump section (29, 29b) of the piston but both the first and second pump chambers (42, 42b; 44, 44b) being in communication with the outlet (36, 36b), wherein a passageway (43, 43b) extends around the middle seal section and provides communication between the first and second chambers (42, 42b; 44, 44b) such that in operation fluid is pumped out of the first chamber (42, 42b) into the passageway (43, 43b) and into the second chamber (44, 44b) before passing out through the outlet (36, 36b), characterized in that: the piston is rotating in the housing, the distal recessed section (13a, 13b) of the pump section (29, 29b) is disposed opposite the pump section (29, 29b) from the transition section (31, 31'), and the flow from the first chamber is routed entirely through the second chamber. a reciprocating piston (10a, 10b) disposed in a pump housing (21, 21b), the housing (21, 21b) comprising an inlet (35, 35b), an outlet (36, 36b), an interior and a middle seal, the piston (10a, 10b) comprising a proximal section (28, 28b) connected to a pump section (29, 29b) at a transition section (31, 31'), the proximal section (28, 28b) being linked to a motor, the proximal section (28, 28b) having a first maximum outer diameter, the pump section having a second maximum outer diameter that is greater than the first maximum outer diameter, the pump section (29, 29b) comprising a distal recessed section (13a, 13b), the pump section (29, 29b) extending between the transition section (31, 31') and a distal end (33, 33b), the pump section (29, 29b) of the piston (10a, 10b) being at least partially and frictionally received in the middle seal of the housing (21, 21b), the housing (21, 21b) and piston (10a, 10b) defining two pump chambers including a first chamber (42, 42b) defined by the distal recessed section (13a, 13b) and distal end (33a, 33b)of the pump section of the piston and the housing (21, 21b), and a second chamber (44, 44b) defined by the transition section (31, 31') and proximal section (28, 28b) of the piston and the housing (21, 21b), a first chamber (42, 42b) defined by the distal recessed section (13a, 13b) and distal end (33a, 33b)of the pump section of the piston and the housing (21, 21b), and a second chamber (44, 44b) defined by the transition section (31, 31') and proximal section (28, 28b) of the piston and the housing (21, 21b), wherein the first and second pump chambers (42, 42b; 44, 44b) are axially isolated from each other by the middle seal and the pump section (29, 29b) of the piston but both the first and second pump chambers (42, 42b; 44, 44b) being in communication with the outlet (36, 36b), wherein a passageway (43, 43b) extends around the middle seal section and provides communication between the first and second chambers (42, 42b; 44, 44b) such that in operation fluid is pumped out of the first chamber (42, 42b) into the passageway (43, 43b) and into the second chamber (44, 44b) before passing out through the outlet (36, 36b), characterized in that: the piston is rotating in the housing, the distal recessed section (13a, 13b) of the pump section (29, 29b) is disposed opposite the pump section (29, 29b) from the transition section (31, 31'), and the flow from the first chamber is routed entirely through the second chamber. the piston is rotating in the housing, the distal recessed section (13a, 13b) of the pump section (29, 29b) is disposed opposite the pump section (29, 29b) from the transition section (31, 31'), and the flow from the first chamber is routed entirely through the second chamber.