How does a piston pump draw fluid into its cylinder?

A piston pump draws fluid into its cylinder through the movement of the piston. As the piston moves outward, the volume available within the cylinder increases, which creates a partial vacuum.12

This reduction in internal pressure allows atmospheric pressure to act on the fluid, forcing it through a one-way inlet valve and into the cylinder. In the context of internal-combustion engines, this specific phase of the cycle is referred to as the suction stroke. Once the cylinder is filled, the piston reverses direction to decrease the available volume, which increases the fluid's pressure and forces it out through an outlet valve.123