To bleed air out of a hydrostatic transmission, follow these steps:
1. Locate the purge valve or plug on the transmission.
2. Start the engine and let it run for a few minutes to warm up.
3. Slowly open the purge valve to release any trapped air. Be cautious as fluid may come out as well.
4. Allow the air to escape until you see only fluid coming out of the valve.
5. Close the purge valve and check the transmission for proper operation.
Fixing Zero Turn Hydrostatic Transmission Problems - Nelson Tractor Blog