Draw cylinder
  In accordance with the mode of action points: single-and   dual-role role 
 Single Cylinder only to Detroit (or plunger) to do one-way movement, pressure that is only leading to a hydraulic cylinder cavity, and the reverse direction movement must rely on external forces (such as from the spring or self-respect, etc.) to achieve;
    Dual role of hydraulic cylinders in the two directions of movement by promoting the oil pressure to achieve.