A pump has a pump barrel formed from a larger diameter section and a smaller diameter section. Each section has a biased piston moveable within the section and the pistons are connected together to form a variable volume chamber between the pistons. As the connected pistons move toward the larger diameter section, a volume of fluid is moved through an inlet valve into the variable volume chamber of increasing volume. When the pistons are moved toward the smaller diameter section, a differential volume of fluid is discharged from the variable volume chamber of decreasing volume through a discharge valve into a discharge conduit. The pistons are actuated to move within the pump barrel by application and release of pressure at a remote end of the discharge conduit.