Yoke actuating piston Compensator valve EDV solenoid (deenergized) Control pressure (P$_{c}$) P$_{case}$ Blocking valve Pump yoke Yoke spring Drive shaft Depressurizing piston P$_{s}$ Case drain P$_{i}$ Outlet high pressure Inlet low pressure (P$_{i}$) Inlet pressure  (P$_{s}$) Outlet pressure  (P$_{c}$) Control pressure  (P$_{case}$) Case pressure EDV pressure Figure 12-32.  Depressurized mode. mechanically controlled by a lever or electrically controlled  by solenoid or servo.  [Figure 12-35] out of the unit. Hydraulic system pressure can be routed  with the selector valve to operate the unit in either direction  and a corresponding return path for the fluid to the reservoir  is provided. There are two main types of selector valves:  open-center and closed-center. An open center valve allows  a continuous flow of system hydraulic fluid through the  valve even when the selector is not in a position to actuate  a unit. A closed-center selector valve blocks the flow of  fluid through the valve when it is in the NEUTRAL or OFF  position.  [Figure 12-33A] Selector valves may be poppet-type, spool-type, piston-type,  rotary-type, or plug-type.  [Figure 12-34]  Regardless, each  selector valve has a unique number of ports. The number  of ports is determined by the particular requirements of  the system in which the valve is used. Closed-centered  selector valves with four ports are most common in aircraft  hydraulic systems. These are known as four-way valves.  Figure 12-33  illustrates how this valve connects to the  pressure and return lines of the hydraulic system, as well as to  the two ports on a common actuator. Most selector valves are  The four ports on a four-way selector valve always have the  same function. One port receives pressurized fluid from the  system hydraulic pump. A second port always returns fluid to  the reservoir. The third and fourth ports are used to connect  the selector valve to the actuating unit. There are two ports  on the actuating unit. When the selector valve is positioned  to connect pressure to one port on the actuator, the other  actuator port is simultaneously connected to the reservoir  return line through selector valve.  [Figure 12-33B]  Thus,  the unit operates in a certain direction. When the selector  valve is positioned to connect pressure to the other port  on the actuating unit, the original port is simultaneously  connected to the return line through the selector valve and  the unit operates in the opposite direction.  [Figure 12-33C] Figure 12-36  illustrates the internal flow paths of a solenoid  operated selector valve. The closed center valve is shown  in the NEUTRAL or OFF position. Neither solenoid is  12-23