Steering doesnt return to center on its own

For your power steering, jack the front up and support on the frame rails, so the steering is free to turn. Turn on the engine and center the steering wheel. Let go and note if the steering box tries to turn it left or right. If so, slightly loosen the 2 bolts on the valve body so you can tap it fwd and aft (bolts are in slotted holes), until it doesn't "self-steer". That centers the spool valve in the valve body. This should be described in service manual. Might be good to first set the sector shaft play as in post #2, insuring it doesn't bind in the center position. I don't think you need to disconnect the Pitman arm like the manual suggests.

The main thing which makes the wheels return to center is how the Steering Angle Inclination (SAI) interacts w/ the tires and weight of the car. SAI is the angle towards or away from the car, of a line thru the centers of the upper and lower ball joints. It is mostly designed-into the car, so isn't adjustable. It does slightly change when you adjust the eccentric bolts in the upper control arm, but you do that to adjust camber and caster, not SAI. Caster is the "shopping cart effect", i.e. how the front wheels tend to follow the vehicle's current direction, but doesn't return the steering to center. Many web articles for more info.