A medical pump (234) for conveying an fluidic medium for example insulin has a housing (230) provided with a piston channel (241) an inlet port (246) extending to the piston channel (241) and configured for connection to a reservoir (226), a piston chamber (250) in fluid flow communication with the channel (241) and an outlet port (236) in fluid flow communication with the piston channel (241) and configured for connection in fluid flow communication with an injection site and a piston (240) located in the piston channel (241) and having a surface for receiving a drive force from a drive shaft (84) wherein the piston (240) is moveable along the longitudinal axis (A2) of the piston channel (241) between fill and dispense positions upon receiving a force from the drive shaft (84). In the fill position, the piston is moved to a position to allow fluid flow communication through the inlet port (246) and into the piston channel (241) and piston chamber (250) and in the dispense position, the piston (240) is moved to a position to obstruct fluid flow communication through the inlet port (246) and to reduce the volume of the piston chamber (250) to force fluid in the piston chamber (250) out of the outlet port (236). A piston channel member (249) within the housing (230) defines the piston channel (241) and the piston chamber (250), the piston channel member (249) having a one-way value (252) which separates the piston chamber (250) an outlet chamber (248) in communication with the outlet port (236). The piston channel member (249) further defines an annular channel (254) around its outer peripheral surface in alignment and fluid flow communication with the inlet port (246), and a flow passage (256) connecting the annular channel (254) and the piston chamber (250), the piston channel member (249) also defining a piston valve seat (258) configured to abut the piston in the piston channel (241) to inhibit fluid back flow from the piston chamber (250) towards the flow passage
