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

The transmission must be purged before operating the tractor for the first time to remove any trapped air that may have developed during shipping. With the engine running at slow throttle on a flat area of about 80 feet, slowly move the motion control lever to full forward for 5 seconds, then full reverse for 5 seconds, and repeat this procedure three times. The transmission should also be purged after removal for service or replacement before operating the tractor. (Page 14)