What is Return to Center (RTC) mechanism in Hydraulic Power Steering System?

In a purely hydraulic power steering system, with no torque on the steering wheel, valve leaves all the ports open, the hydraulic fluid pressure very low everywhere and the piston can move reasonably freely. In effect, the power steering does nothing and does not resist being moved.

All vehicles have some castor angle, so that the front wheels tend to return to the straight-ahead position. The two front wheels would each do that separately if they weren't joined by the track rod or steering rack. It's the same effect that makes the wheels on supermarket trolley steer in the direction the trolley is pushed.

So the steering geometry supplies a small force to push the steering system back to centre, and the hydraulic system doesn't resist that force significantly if there is no torque on the steering wheel from the driver.

just a dumb question or statement. All of your posts have something to do with steering, if you don't understand the simple mechanics of vehicle steering how can you be trusted to do more complex things with or to it? I sure hope these questions you keep posting are some type of school work, not real design work.