Regardless of the machine size, brand, or hydraulic structure—even for gear pumps—it is crucial to remove any air trapped in the hydraulic system after maintenance or component replacement.

This step is very important. For example, a CAT 330C excavator suffered three consecutive new gear pump failures in winter because the hydraulic oil was too viscous to properly purge air, causing the pumps to run dry and fail.

To prevent such incidents, I have trained maintenance personnel on the principles, precautions, and lessons learned, and incorporated proper tools and processes to ensure air is removed from the system after maintenance.

Key Steps

1. Always bleed air after replacing hydraulic oil or system components.

2. Start from the system’s lowest point:

   • Begin at the pump inlet.

   • Disconnect high points in the return lines.

   • Fill the system with hydraulic oil.

   • Use compressed air at the oil tank inlet to push oil through the system until the pump return line flows continuously without bubbles.

3. Operate the system carefully:

   • Use low engine throttle to slightly lift the boom to its highest position.

   • Shut down the engine and lower the boom to the ground (this step pressurizes the tank; older machines can skip this, but the lifting step must still be done).

   • Without shutting off the engine, move each hydraulic cylinder in both directions repeatedly, stopping 10–15 cm before the end positions, then move to the end.

   • This ensures trapped air is expelled from the system.

4. Troubleshooting:

   • If air keeps appearing after bleeding, it indicates a loose connection or improper sealing somewhere in the system—inspect and fix the source.

5. Oil Level Check:

   • Follow the machine manufacturer’s instructions. There is no universal standard.

Summary:
Bleeding air from a hydraulic system is a simple but often overlooked step. Proper attention prevents pump damage, ensures smooth operation, and avoids repeated failures.