US Patent 7758321 Pump apparatus

A pump apparatus comprises a piston, which is provided displaceably along a first chamber formed in a body under the action of a pilot pressure, an indirect medium composed of a non-compressive fluid and which is to be pressed by the piston, and a diaphragm that is flexibly bendable in cooperation with the indirect medium. When the fluid is discharged from a discharge port, a change in volume caused by the displacement of the piston in the axial direction is identical to the change in volume caused by the displacement of the diaphragm in the axial direction.