Understanding the Importance of Bleeding Hydraulic Cylinders

Air in a hydraulic system can compress, unlike hydraulic fluid, which leads to sponginess in the system’s response, decreased efficiency, and uneven power transmission. Bleeding the system removes this trapped air, restoring the hydraulic system’s precision and power.

Symptoms Indicating the Need for Bleeding

Before proceeding, it’s important to recognize the signs that indicate air is present in the hydraulic system:

  • Spongy or Inconsistent Response: The most noticeable symptom, indicates the presence of air in the system.
  • Decreased Power Output: The system doesn’t seem as powerful or effective as it should be.
  • Erratic Cylinder Movement: The hydraulic cylinder moves unevenly or jerks.
  • Noise: Unusual noises from the hydraulic pump, such as whining or knocking.

Tools and Preparation

Gather the necessary tools before starting:

  • Wrenches (suitable for your system’s connections)
  • Hydraulic fluid (as specified by your system’s requirements)
  • Clean rags or towels
  • Safety gear (gloves, safety glasses)
  • A container for catching any excess fluid
  • A helper (for some methods)

hydraulic cylinder
Ensure the system is off and pressure is released to avoid any risk of injury or damage.

Step-by-Step Guide to Bleeding a Hydraulic Cylinder

  1. Identify Bleed Valves: Locate the bleed valves on your hydraulic cylinder. These are typically found at the highest points in the system where air is likely to accumulate.
  2. Prepare the Area: Clean the area around the bleed valves to prevent contamination. Lay down rags or towels to catch any spilled fluid.
  3. Position the Cylinder: If possible, position the cylinder so the rod is fully retracted. This helps in pushing the air towards the bleed valves.
  4. Open the Bleed Valves: Using a wrench, carefully open the bleed valves. Do not fully remove them; just loosen enough to allow air and fluid to escape.
  5. Activate the Hydraulic System: Turn on the hydraulic system. This will pump fluid through the system, forcing air out through the bleed valves.
  6. Monitor Fluid and Air Discharge: Watch for a steady stream of hydraulic fluid free of air bubbles. Initially, you might see a mix of air and fluid coming out.
  7. Work the Cylinder: Extend and retract the cylinder several times. This helps to move any trapped air towards the bleed valves.
  8. Close the Bleed Valves: Once you see a steady stream of fluid without air bubbles, close the bleed valves. Make sure they are securely tightened to prevent leaks.
  9. Check Fluid Levels: After bleeding the system, check the hydraulic fluid reservoir. Top it up if the level has dropped due to the bleeding process.
  10. Clean Up: Clean any spilled fluid and dispose of the rags or towels used in the process.
  11. Test the System: Run the hydraulic system and observe the cylinder’s operation. It should now work smoothly without any sponginess or erratic movements.

hydraulic cylinder

Additional Tips

  • Refer to the Manufacturer’s Manual: Always consult the hydraulic system’s manual for specific instructions related to bleeding.
  • Use the Correct Fluid: Ensure you use the hydraulic fluid type recommended by the manufacturer.
  • Avoid Over-tightening: When closing the bleed valves, avoid over-tightening as it can damage the valves.
  • Safety First: Always follow safety procedures. Wear gloves and safety glasses to protect against hydraulic fluid, which can be harmful if it contacts skin or eyes.

Regular Maintenance

Regular maintenance, including periodic bleeding of the hydraulic system, is essential to keep it functioning optimally. Establish a maintenance schedule based on the manufacturer’s recommendations and the system’s usage.

Troubleshooting Common Issues

  • Persistent Air in the System: If air continues to be a problem, check for leaks in the system, as they can allow air to enter.
  • Difficulty in Bleeding: Some systems may require special procedures or a specific order in which bleed valves should be opened.


Bleeding a hydraulic cylinder is a vital maintenance task that ensures the efficiency and longevity of your hydraulic system. By carefully following the steps outlined above, you can effectively remove air from the system, restoring its smooth and efficient operation. Regular maintenance, including proper bleeding, will help prevent potential issues and downtime, ensuring that your hydraulic system remains reliable and effective for its intended applications.