When the piston down in weight under the action, the hydraulic control valve to open, and no inferior back pressure, is likely to close the free falling, falling rapidly, the pump can fill the hydraulic cylinder cavity, to reduce the cylinder pressure on the chamber, and even produce a vacuum, a hydraulic control one-way valve closed due to pressure drop control. Once the gear pump check valve is closed, the control pressure is raised again, and the check valve is opened again, and the piston of the oil cylinder begins to decrease again. As the volume of the tube is also involved, this phenomenon is usually low frequency oscillation.

This phenomenon can be met in use of bushing pumps.