Purging is done when you have put everything back together.
1. Raise rear wheels off ground. Brace with jack stands.
2. Pull bypass valves so it freewheels.
3. Get on mower start it and release the brake.
4. Push drive lever full forward for 10 seconds the full reverse for 10 sec. Do the 10 times for forward and 10 times for reverse alternating between each.
5. Return levers to the parked position and set brake. Turn off the mower. Move bypass valve bake to normal operation.
6. Get back on the mower and start it. Do step #4 again.
7. Return levers to the parked position and set brake. Turn off the mower.
8. Return mower to ground.
9. Test for proper working. If not repeat.
10. Check oil level and fill.