Pump Relief valve Press port Cyl 1 port Thermal relief valve Selector valve (off) Selector valve  (extend) Selector valve  (retract) Cyl 2 port Return port Reservoir A B C Figure 12-33.  Operation of a closed-center four-way selector valve, which controls an actuator. Figure 12-34.  A poppet-type four-way selector valve.  Figure 12-35.  Four-way servo control valve. applied on the left end of the spool. The center lobe of the  spool no longer blocks system pressurized fluid, which flows  to the actuator through the left actuator line. At the same time,  return flow is blocked from the main spool left chamber so  the actuator (not shown) moves in the selected direction.  Return fluid from the moving actuator flows through the  right actuator line past the spool and into the return line.  [Figure 12-37] energized. The pressure port routes fluid to the center lobe  on the spool, which blocks the flow. Fluid pressure flows  through the pilot valves and applies equal pressure on both  ends of the spool. The actuator lines are connected around  the spool to the return line. When selected via a switch in the cockpit, the right solenoid is  energized. The right pilot valve plug shifts left, which blocks  pressurized fluid from reaching the right end of the main  spool. The spool slides to the right due to greater pressure  Typically, the actuator or moving device contacts a limit  switch when the desired motion is complete. The switch  causes the right solenoid to de-energize and the right pilot  valve reopens. Pressurized fluid can once again flow through  12-24