Abstract: An improved pump, reservoir and reservoir piston are provided for controlled delivery of fluids. A motor is operably coupled to a drive member, such as a drive screw, which is adapted to advance a plunger slide in response to operation of the motor. The plunger slide is removably coupled to the piston. The piston comprises a first member and a second member. The first member has an external proximate side and an external distal side. The external proximate side is adapted to contact the fluid and is made of a material having a first stiffness. The second member has a first side and a second side and is at least partially disposed within the first member. The first side of the second member is adjacent to the external proximate side of the first member and is made of a material having a stiffness which is greater than the first stiffness.