How do I purge the transmission and why is it necessary?

Purging removes trapped air from the hydraulic drive system that may have developed during shipping. With the freewheel control disengaged and the engine running at slow throttle, move the motion control lever to full forward for five seconds, then full reverse for five seconds, repeating three times. Then re-engage the transmission and repeat the process with the tractor actually moving approximately five feet forward and five feet in reverse, three times. (Page 14-15)