If you have replaced the main piston ( # 60102-00 for water softeners ) and the brine valve piston assembly ( # 60032 ), and your "freely" turning valve still gets stuck, then it is time to take a close look at the Timer Gear Retaining Pin. The pin does not show in most Manuals.

It is this pin which keeps the motor gear engaged on the Idler Gear ( part # 13017 ). Check the condition of this pin and the Idler Gear.
You will only need to remove the timer motor to "see" if the pin is broken or missing.

If the pin has broken off ( the plastic holding it in place sometimes fails ), then you can drill a small hole and insert a small "finishing" nail cut just long enough to fit inside the motor gear. A little PVC cement helps push the new "pin" in through the drilled hole, and helps hold it in place. This is a method I have used several times with success.
Otherwise, you can order a new empty housing ( part # 14448-000 ), or a complete Power Head ( part # 60354 with 4 styles to choose from ).