The piston pump
When the piston moves upward, the liquid under atmospheric pressure enters the tube, lifts the bottom valve and moves behind the piston.
When the piston moves downward, the liquid under the piston presses on the bottom valve and it closes.
At the same time, the pressure of the fluid opens the valve inside the piston, and this fluid moves into the space above the piston.
When the piston moves upward, the liquid under atmospheric pressure enters the tube, lifts the bottom valve and moves behind the piston.
When the piston moves downward, the liquid under the piston presses on the bottom valve and it closes.
At the same time, the pressure of the fluid opens the valve inside the piston, and this fluid moves into the space above the piston.