How to Bleed a Hydraulic Cylinder

January 4, 2024
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From construction equipment to automobile parts, hydraulic cylinders are essential. Like any fluid dynamics system, hydraulic cylinders work best when the fluid inside is free of air. However, over time or due to certain operational issues, air can find its way into the system. This trapped air can lead to various problems and inefficiencies, making the process of bleeding (or purging the hydraulic system of this unwanted air) a critical maintenance task.

Why Bleeding is Crucial for the Optimal Function of Hydraulic Systems:

At its core, a hydraulic system leverages the incompressibility of fluids to transmit power. Air, however, is compressible. When air bubbles are present in the hydraulic fluid, they can compress under pressure, leading to spongy or unresponsive behavior in the machinery. This not only affects the efficiency but can also result in imprecise movements and potential control issues. Therefore, for the hydraulic cylinder to deliver consistent power and function as designed, it’s essential to remove any trapped air.

Accumulation of Air in Hydraulic Systems and Its Consequences:

There are various reasons air might enter a hydraulic system:

  • Leakages: Any small leaks in the system, especially at seals or connection points, can allow air to seep in.
  • Fluid Changes or Additions: When adding or changing the hydraulic fluid, air can get introduced if not done carefully.
  • Temperature Fluctuations: As hydraulic fluid temperatures change, dissolved air can be released into the system.

The presence of this air leads to several adverse effects:

  • Decreased Efficiency: Air reduces the hydraulic system’s ability to transmit power effectively.
  • System Noise: Air bubbles can lead to knocking or gurgling sounds.
  • Component Wear: The compressibility of air can cause shock waves in the system, leading to increased wear and tear on components.

Given these consequences, it becomes clear why regularly checking for and removing air from hydraulic cylinders is of utmost importance. The process of bleeding ensures the longevity and optimal performance of the equipment in which the hydraulic cylinder operates.

Signs Your Hydraulic Cylinder Needs Bleeding

Hydraulic systems, when operating efficiently, tend to be smooth and consistent in their performance. However, when air infiltrates these systems, it can manifest in multiple noticeable ways, undermining the operation and potentially leading to further complications. Being vigilant and recognizing the symptoms of trapped air is the first step to maintaining a healthy hydraulic system.

Decreased Efficiency or Slow Operation:

One of the foremost indications of air being present in a hydraulic cylinder is the system’s diminished efficiency. Since hydraulic systems rely on the incompressibility of fluids to transmit force and power, the presence of compressible air disrupts this dynamic. This results in the machinery working harder than necessary to achieve the same results, leading to slower operation. In practical terms, this might be seen as a hydraulic arm moving more sluggishly than usual or a hydraulic press taking longer to complete its cycle.

Unsteady or Jerky Movements:

Air bubbles in the hydraulic fluid can cause inconsistent resistance. As the piston in the cylinder moves, it might encounter pockets of air, leading to sudden drops in resistance followed by increased resistance once the fluid takes over again. This results in the jerky or unsteady movements of machinery. For instance, a hydraulic lift might not raise a platform in a smooth manner, instead showing moments of stuttering or hesitations.

Noise During Operation:

A well-maintained hydraulic system should operate relatively quietly, with only the sound of the machinery in action. However, when air gets trapped inside, it can lead to distinctive noises. As the compressible air bubbles get forced through the system under pressure, they can cause knocking, gurgling, or whining sounds. These noises are not just bothersome; they are a clear indication that there’s unwanted air in the system that needs to be addressed.

Being attuned to these signs is essential for anyone responsible for the upkeep of hydraulic machinery. The quicker one identifies and rectifies the issue, the less wear and tear the equipment experiences, ensuring a longer operational life and reduced maintenance costs.

Tools and Materials Required

Before beginning the process of bleeding a hydraulic cylinder, it’s essential to gather the appropriate tools and materials to ensure the procedure goes smoothly and safely. Having everything at hand not only speeds up the process but also minimizes the risk of contamination and accidents.

Clear Container or Bucket for Collecting Hydraulic Fluid:

When bleeding the system, old hydraulic fluid and any trapped air will be expelled from the cylinder. A clear container or bucket is crucial for collecting this fluid to prevent environmental contamination and make the process less messy. Additionally, using a transparent container helps in observing the air bubbles being released, giving a clear indication of the bleeding progress.

Hydraulic Fluid:

After bleeding a hydraulic cylinder, it’s often necessary to top up the system with fresh fluid. Always ensure you’re using the appropriate type and grade of hydraulic fluid as specified by the equipment’s manufacturer. Using the wrong fluid can lead to inefficiencies and potential damage.


Wrenches are indispensable for opening and closing bleed valves or any other fittings on the hydraulic system. Having a set of wrenches ensures that you have the right size for every component, minimizing the risk of damaging the nuts or bolts.

Rags or Absorbent Pads:

Despite all precautions, the bleeding process can get a little messy with potential fluid spills. Having rags or absorbent pads handy helps in immediately cleaning up any spills, preventing slip hazards and potential fluid contamination.

Safety Gear (Gloves, Safety Glasses):

Safety should always be the priority when working with hydraulic systems. Wearing gloves protects the hands from hydraulic fluid, which can sometimes be harmful if it comes into prolonged contact with the skin. Safety glasses shield the eyes from any inadvertent fluid splashes, ensuring your eyes remain safe throughout the process.

Equipped with the right tools and materials, you’re now set to safely and efficiently bleed the hydraulic cylinder. It’s always a good practice to double-check that you have everything you need before starting, as interruptions can lead to complications or extend the duration of the task.

Safety Precautions Before Bleeding

Safety should always be paramount when working with hydraulic systems. Before embarking on the bleeding process, it’s essential to observe certain precautions to ensure that the operation goes smoothly without risking harm to oneself or damage to the equipment. Here are the critical safety measures to keep in mind:

Ensure the System is Not Under Pressure:

Before starting the bleeding process, always ensure that the hydraulic system is depressurized. Working on a pressurized system can lead to sudden releases of hydraulic fluid, potentially causing injury. You can depressurize the system by shutting off the machine or equipment and then slowly and carefully releasing any existing pressure using the system’s control valves.

Work in a Well-Ventilated Area:

Hydraulic fluids can sometimes emit fumes that are not only unpleasant to smell but can also be harmful when inhaled in large quantities. Working in a well-ventilated space ensures that any emitted fumes are quickly dispersed, reducing the risk of inhalation and ensuring a safer work environment.

Always Wear Safety Gear:

When working with hydraulic systems, it’s always recommended to wear appropriate safety gear. This includes safety glasses to protect the eyes from accidental splashes and gloves to protect the hands from hydraulic fluid, which can be harmful or irritating upon contact. If there’s a risk of fluid splashing, consider wearing a protective apron or overalls.

Keep the Area Clean to Prevent Contamination:

Cleanliness is vital when working with hydraulic systems. Any contaminants, such as dirt or debris, can severely compromise the system’s efficiency and potentially cause damage. Before beginning the bleeding process, clean the area around the bleed valves and other components. It’s also a good practice to clean up any spills immediately to prevent slip hazards and further contamination.

Remember, safety precautions are not just for personal protection but also to ensure the longevity and efficient operation of the hydraulic system. Always prioritize safety, and when in doubt, refer to the equipment’s user manual or consult with an expert.

Step-by-Step Bleeding Process

Bleeding a hydraulic cylinder is a straightforward process but requires careful attention to detail to ensure it’s done correctly. Here’s a step-by-step guide to help you bleed your hydraulic cylinder effectively:

1. Locate the Bleed Valve

Before you begin the bleeding process, you must first locate the bleed valve on your hydraulic cylinder. The bleed valve is typically a small valve or screw, often found at the topmost part of the cylinder or near the hose connections, depending on the design of the cylinder.

  • Identify the Valve: Start by examining the hydraulic cylinder. Look for any nozzles or small screws labeled as “bleed” or “vent.” If you’re uncertain, refer to the cylinder’s user manual or diagrams.
  • Ensure Accessibility: Make sure the bleed valve is easily accessible. You may need to reposition the cylinder or remove obstructions around it to access the valve comfortably.

2. Preparing the Cylinder

Before you start bleeding a hydraulic cylinder, it’s essential to ensure the system is safe and the cylinder is correctly positioned.

  • Depressurize the System: As mentioned in the safety precautions, make sure the hydraulic system is not under pressure. Shut down any equipment or machinery, and use the control valves to release any residual pressure.
  • Position the Cylinder: Ideally, the cylinder should be positioned so the bleed valve is at its highest point. This ensures that any trapped air will naturally rise and can be released through the bleed valve.
  • Gather Your Tools: Have all your tools and materials, such as the clear container or bucket, wrenches, and rags, within arm’s reach. This will help streamline the process and ensure you have everything you need on hand.
  • Wear Safety Gear: Put on your safety glasses and gloves to protect yourself from any hydraulic fluid that might splash during the process.

Now, with everything in place, you are ready to begin the bleeding process.

3. Open the Valve

One of the critical steps in the bleeding process is to open the bleed valve correctly. Here’s how to do it:

  • Position the Container: Before opening the valve, place your clear container or bucket beneath the bleed valve to catch any hydraulic fluid that may come out. This prevents any mess and also allows you to observe the fluid’s condition, which can be an indicator of system health.
  • Slowly Turn the Valve: With your wrench, gently turn the bleed valve counterclockwise. Start by opening it slightly. You should hear a hissing sound, indicating the release of air.
  • Observe Fluid Flow: As you continue to open the valve, hydraulic fluid will begin to flow out. Pay attention to the fluid’s consistency. Initially, it might come out as a mixture of fluid and air bubbles, which is a sign that the air is being expelled.

Note: Be patient and avoid opening the valve too rapidly, as this can cause hydraulic fluid to spray out, potentially leading to injury.

4. Release the Air

Trapped air within the hydraulic cylinder can reduce its efficiency and lead to system malfunctions. Here’s how to ensure all the air is released:

  • Watch for Air Bubbles: As the hydraulic fluid flows out, watch for air bubbles in the fluid. The presence of bubbles indicates that there’s still air trapped in the system.
  • Wait for Steady Flow: Continue to let the fluid flow until you see a steady stream without any bubbles. This means that most, if not all, of the trapped air has been expelled from the system.
  • Tap the Cylinder: Sometimes, tapping the sides of the cylinder lightly with a rubber mallet can help release any stubborn air pockets. Do this gently, and be sure not to damage the cylinder.
  • Close the Valve: Once you are confident that all the air has been released and the fluid coming out is bubble-free, slowly turn the bleed valve clockwise using your wrench to close it. Ensure it’s tightly sealed to prevent any future leaks or air ingress.
  • Check System Pressure: After bleeding, it’s a good idea to check the hydraulic system’s pressure to ensure it’s operating at the correct levels. If the pressure is too low, you may need to add more hydraulic fluid.

Remember, while bleeding is a simple procedure, it’s essential to perform it carefully and methodically to ensure the optimal functioning of your hydraulic cylinder.

5. Close the Valve and Refill

Bleeding a hydraulic system can lead to a loss of hydraulic fluid, so it’s essential to close the system correctly and refill it to its operational level. Here’s how:

  • Closing Without Air Entry: Begin by observing the hydraulic fluid flow. Once you’re confident that all the air bubbles are out and you have a steady stream of hydraulic fluid, it’s time to close the bleed valve. Slowly turn the valve clockwise using your wrench. As you do this, ensure that no external air gets sucked back into the system.
  • Check Fluid Level: After closing the bleed valve, check the hydraulic fluid reservoir. It will likely be lower than the optimal level due to the fluid lost during the bleeding process.
  • Refill the System: Using a funnel to avoid spills, slowly pour hydraulic fluid into the reservoir until you reach the indicated fill line or mark. Make sure to use the correct type of hydraulic fluid recommended for your specific system.
  • Reinstall Any Caps or Covers: If your hydraulic system has a reservoir cap or any protective covers that were removed for the bleeding process, be sure to reinstall them securely.

6. Testing the System

After the bleeding process, it’s imperative to test the hydraulic cylinder to ensure that the bleeding was successful and the system operates correctly.

  • Start Slowly: Initially, operate the hydraulic system at a reduced pace. This allows you to observe its performance without placing undue stress on it.
  • Observe for Jerky Movements: One of the main reasons for bleeding a hydraulic system is to eliminate unsteady or jerky operations. As you test, check if these issues are resolved.
  • Listen for Noise: Air in the system can cause a gurgling sound. After bleeding, such noises should diminish or be entirely absent. If you still hear unusual sounds, you might need to repeat the bleeding process.
  • Check for Leaks: Always look for any signs of hydraulic fluid leaks around the bleed valve, seals, and other components. If you find any, address them immediately.
  • Monitor Pressure: Use any pressure gauges or indicators your system may have. The pressure should remain steady and within the recommended range.

If all looks and sounds good, and the hydraulic cylinder operates smoothly, the bleeding process has been successful. Regular maintenance, including periodic checks and bleeds if necessary, will ensure the longevity and efficient operation of your hydraulic system.

Troubleshooting After Bleeding

Even after going through the bleeding process, there are instances where the hydraulic cylinder might not function as expected. Here’s a guide on troubleshooting post-bleeding issues:

  • Recheck for Air Bubbles: Sometimes, not all the air is expelled in the first bleeding process. Consider bleeding the system once more, ensuring that all steps are followed carefully.
  • Inspect for Damaged Seals: Damaged or worn-out seals can allow air to re-enter the system. Check all seals and replace any that appear compromised.
  • Check Hydraulic Fluid Quality: The hydraulic fluid itself might be the issue. Contaminated or old fluid can affect system performance. It might be time for a complete fluid change.
  • Assess the Pump: The hydraulic pump might be drawing in air due to worn components or seal issues. Examine the pump and consider servicing if required.
  • Evaluate Valves and Connections: Loose connections or malfunctioning valves might be the culprits. Ensure that all connections are tight and that valves are functioning correctly.
  • System Pressure: If the hydraulic system’s pressure is too high or too low, it can cause operational issues. Check the system’s pressure and adjust as necessary.

Maintenance Tips to Prevent Air Entrapment

Regular maintenance can prevent many common hydraulic cylinder issues, including air entrapment. Here are some essential maintenance tips to consider:

  • Regular System Checks: Periodically inspect your hydraulic system, paying attention to potential leak points, the quality of the hydraulic fluid, and the overall operation of the cylinder.
  • Seal Maintenance: Seals are crucial for preventing air from entering the system. Always use high-quality seals and replace them at the first sign of wear or damage.
  • Proper Fluid Levels: Always maintain the hydraulic fluid at recommended levels. A low fluid level can cause the pump to draw in air, leading to air entrapment.
  • Avoiding Contamination: Keep the hydraulic fluid clean. Regularly check and, if necessary, replace the hydraulic fluid filters.
  • Regular Fluid Changes: Over time, hydraulic fluid can degrade or become contaminated. Adhering to a regular schedule for changing the fluid can prevent many operational issues.
  • Using the Right Fluid: Always use the manufacturer-recommended hydraulic fluid. Different systems require different fluid types, and using the wrong one can lead to operational issues.
  • Tight Connections: Loose connections can draw in air. Regularly check and tighten all hydraulic system connections.

By adhering to these maintenance guidelines and performing regular checks, you can ensure the longevity of your hydraulic system and avoid issues related to air entrapment.

Frequently Asked Questions about Bleeding Hydraulic Cylinders

Why is bleeding a hydraulic cylinder necessary?

Bleeding removes trapped air from the hydraulic system. Air can compress, unlike hydraulic fluids, leading to decreased efficiency, spongy operations, and even potential system damage.

How often should I bleed my hydraulic cylinder?

While there’s no fixed schedule, you should bleed the cylinder whenever you notice signs of air entrapment, like jerky movements, noise during operation, or decreased efficiency. Regular maintenance checks can help identify the need.

Can trapped air in the system damage the hydraulic cylinder?

Yes, trapped air can lead to cavitation, increased wear and tear, and can compromise the system’s efficiency and lifespan.

How do I know if I’ve successfully bled all the air out?

After bleeding a hydraulic cylinder, test the system. Smooth operation without noises or jerks typically indicates successful bleeding. However, if issues persist, consider repeating the bleeding process or checking for other system issues.

What are the dangers of not wearing safety gear during the bleeding process?

Hydraulic fluid can be harmful if it comes into contact with skin or eyes. Additionally, pressurized fluid can eject forcefully, potentially causing injury. Safety gear minimizes these risks.

What hydraulic fluid I should use when refilling the system?

Always use the hydraulic fluid recommended by the manufacturer. Different systems require different fluid types, and using an incorrect one can lead to operational issues or system damage.

I’ve bled the system, but the cylinder is still malfunctioning. What should I do?

There could be multiple reasons for malfunctioning, including damaged seals, contaminated fluid, or issues with the pump. Consider troubleshooting based on the symptoms or consult with a hydraulic system specialist.

Can I bleed a hydraulic cylinder without a bleed valve?

While bleed valves make the process simpler, it’s possible to bleed a system without one, though it might be more complex. Always refer to the manufacturer’s guidelines or seek expert advice.

Why is my hydraulic fluid foamy after bleeding?

Foamy hydraulic fluid indicates the presence of air. If the fluid remains foamy after bleeding, it suggests that not all the air was expelled. Consider re-bleeding the system.

Is it normal for some hydraulic fluid to be lost during bleeding?

Yes, a small amount of hydraulic fluid loss is normal during the bleeding process. Always have additional fluid on hand to refill the system as needed.

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