How to Bleed Air from a Hydraulic Cylinder?

Hydraulic cylinders are an essential part of several machines. These include excavators, forklifts, tractors, and industrial equipment. These cylinders rely on hydraulic fluids to work smoothly. However, sometimes air can get trapped inside the hydraulic cylinder, causing problems. So the question is: How to bleed air from a hydraulic cylinder? Remember, even small air bubbles can cause serious performance problems.

For example, you might notice jerky movements, slow operation, and strange noises. All these issues ultimately damage the entire system over time. That’s why bleeding the air becomes crucial. But many beginners become confused at this point. Hence, to help you get it, I will discuss a step-by-step process for air bleeding from a hydraulic system in this guide.

Before starting anything, you must first understand why the air enters the hydraulic cylinder. When you know the reason, it will later be easier to bleed that air and prevent it from happening again. Here are a few reasons why air can enter a hydraulic cylinder.

• During Installation: When you install the hydraulic cylinder for the first time, it’s very common for air to enter. Actually, the cylinder and pipes remain empty before they are filled with hydraulic fluid. Now, when you install a cylinder and fill it with fluid, the small air pockets also get trapped.
• Low Hydraulic Fluid Level: Hydraulic cylinders need an adequate amount of fluid to operate correctly. Now, when the fluid level becomes too low, the pump starts pulling air along with the fluid. This air spreads throughout the entire hydraulic system, causing serious problems.
• Loose Connections: Another reason for air entering is a loose connection of the cylinder. As you know, a hydraulic system contains many joints and fittings. If any of those connections loosen, the air quickly seeps in through that small gap. This makes the problem harder to notice at first. However, over time, it can damage the cylinder.
• Maintenance Work: Maintenance is very crucial for hydraulic cylinders. So when you perform the maintenance, i.e., repair or replace the hose, seal, or connection, the cylinder is exposed to air. After maintenance, the trapped air will remain inside.
• Worn Seals: No doubt seals keep the hydraulic system closed and prevent leaks. However, over time, these seals become weak, cracked, or damaged due to wear and tear. So from these worn-out or weak points, air can easily get trapped.

Now you know the common reason why air enters the hydraulic cylinder. Next, it comes to cope with this issue. At first, it seemed very technical to bleed air from the hydraulic cylinder. But here, I will discuss some steps to help you bleed air from the hydraulic cylinder.

The very first step while bleeding the air is to check the level of hydraulic fluid in the system. As I said above, hydraulic systems entirely depend on the hydraulic fluid. If there is not enough fluid, the system cannot build the right pressure. As a result, even if you do everything to bleed the air, some air bubbles will remain in the cylinder. So first, locate the hydraulic fluid tank or reservoir in your hydraulic system.

Usually, machines have clear markings indicating the minimum and maximum fluid levels. Look at the level carefully. If it is below the recommended mark, you need to add more hydraulic fluid before doing anything else. However, at this point, you should also be careful about using the right type of hydraulic fluid. Using the wrong fluid type will stop the entire system from working.

Are you sure the hydraulic fluid level is correct? The next crucial step is to find the point where the trapped air will actually escape. Most hydraulic cylinders come with a special bleed valve. You will likely find this bleed valve near the top of the cylinder because air naturally rises. You have to look carefully around the cylinder body and locate that valve. However, it is best to consult your machine’s manual to find the valve.

The bleed valve is usually small and may look like a screw or nipple. But what if your cylinder does not have a dedicated bleed valve? Don’t worry. In that case, you can use a hose connection or fitting as an alternative. However, when locating the fitting, choose one at the highest point of the system. Why? Because this is where air usually collects.

Once you have located the bleed valve or fitting, carefully loosen it. This is the step that will finally allow the trapped air to escape from the hydraulic cylinder. But at this stage, you must know that you do not need to remove the valve completely. Instead, take a wrench and turn the bleed valve slowly. Keep turning until it just opens a little, not too much.

What if you open it too much? Definitely, hydraulic fluid will spill quickly. This will, in turn, create a mess or even pose a safety risk to you and the cylinder. That’s why a small opening is enough to let the air pass through. However, while doing this, I suggest that you place a container or cloth under it. This will catch any fluid that comes out along with the air.

After you slightly loosen the bleed valve, the next very critical step is to operate the hydraulic cylinder. But at this point, many rush the process and make mistakes. However, this should be done slowly and in a controlled way. Why? Because the aim is to push the trapped air bubbles toward the escape point, not the fluid. So start the machine and gently move the cylinder through its normal motion.

When the cylinder moves, the hydraulic fluid begins to flow. This fluid flow directly pushes the trapped air toward the open bleed valve. As you continue this, you will notice that air begins to escape from the valve along with a little fluid. At first, you will see that the cylinder’s movement is uneven or jerky because air is still inside. Don’t worry, it’s normal. But as air starts to come out, you will notice that the jerks decrease.

Now, when the hydraulic cylinder is operating, watch for air bubbles. At the beginning, you will usually see a mix of hydraulic fluid and small air bubbles coming out. These bubbles mean air is still present inside the cylinder. This is normal at this stage, so there is no need to worry. So keep the cylinder running slowly and observe the airflow.

As time passes, you will see that the number of bubbles is reducing. It means that all the air is finally leaving the hydraulic cylinder, and there are no jerks. However, at this point, many beginners rush and close the valve quickly. Remember that’s totally the wrong practice. If you stop before all the bubbles are gone, some air may remain. This later causes problems and requires rebleeding the air.

You are sure that all the air is gone and there are no jerks in the hydraulic cylinder. Right? It’s finally time to close the bleed valve. For this, take the wrench again and slowly tighten the valve. Make sure it is secure, but do not over-tighten it. Why? Because excessive force can damage the valve or its threads. Once the valve is fully closed, it is time to test the hydraulic cylinder.

Start the machine and operate the cylinder normally. Move it through its full range—extend and retract it a few times. This will help you check whether air bubbles are released. During testing, also pay attention to how the cylinder moves. Remember, the cylinder should move smoothly and steadily. There should be no jerking, slow response, or unusual noise. If that’s the case, it means you have bled the air from the hydraulic cylinder.

Once air enters the hydraulic cylinder, a series of problems will follow. So it is best to prevent that air from entering the hydraulic cylinder. But how? Here are some useful tips that will help you resolve this issue from the start.

As I said above, the main reason behind the air trap in the cylinder is a low fluid level. Due to this low level, the hydraulic pump quickly starts pushing the air along with the fluid. To prevent this, always monitor the fluid level in the reservoir. Make it a habit to check it regularly, especially before using the machine for long work. If you notice the level is low, refill it with the manufacturer-recommended type of hydraulic fluid.

Even a small leak in the hydraulic system can allow air to slowly enter over time. Right? These leaks often happen in hoses, joints, fittings, or connectors. So look closely at all these parts of the system and check for signs of oil seepage and wet spots. If you find any issues, fix them immediately instead of ignoring them.

Seals play a key role in keeping the hydraulic system airtight. If that seal is not of good quality, it will wear out quickly and create weak points in the cylinder. From these points, the air can quickly get trapped. So it is best to buy a high-quality, durable seal that lasts longer and wears out less quickly.

Air problems often start right from installation. If a hydraulic cylinder or any connected parts are not installed correctly, small gaps will form. These gaps allow air to enter later. So, always follow the proper installation steps. Make sure all connections are tight, aligned, and fitted correctly.

Lastly, regular maintenance is the best way to prevent air from entering the cylinder. Over time, parts naturally wear out, and the fluid becomes dirty, often without you noticing. But when you regularly maintain your hydraulic scythe, these issues don’t arise, and air doesn’t get trapped.

Do all cylinders have bleed valves?

No, not all hydraulic cylinders have bleed valves. Some cylinders come with a built-in bleed valve. However, other cylinders also require loosening a fitting to remove air.

Can air enter again after bleeding?

Yes, air can re-enter if there is a leak, a low fluid level, or during maintenance work. That’s why you should regularly check the system and use high-quality and proper sealing.

How often should you bleed the system?

You don’t need to bleed the air regularly. Only bleed the system after installation, after repair, or when you notice signs of air (such as jerky movement or weak performance).

Let’s conclude. Hydraulic systems have become an important part of several modern machines and equipment. These systems use hydraulic cylinders that carry hydraulic fluid. However, sometimes air can get trapped in the cylinder, leading to serious issues later. For example, the air reduces the system’s efficiency, causes jerks, or completely stops the machine.

That’s why removing that air is very crucial. But many don’t know how. Therefore, in this guide, I have explained some steps to help you bleed the air from the hydraulic cylinder. But remember, all those steps should be performed carefully. A slight mistake will cause the problem again. However, the best practice is to stop the air from entering the cylinder from the start. For this, you can take some precautions that I discussed above.