Bleeding a Hydraulic Cylinder

Kappa Engineering Bleeding a Hydraulic Cylinder

Bleeding a Hydraulic Cylinder

Hydraulic cylinders make light work of difficult tasks. They provide the force in machinery that shapes the modern world. 

Issues such as pockets of air entering the hydraulic cylinder chamber will result in your cylinders not working as efficiently as they should. This is why it is necessary to bleed your hydraulic cylinders. If your hydraulic systems are not bled it might even lead to catastrophic system failure. If you don’t want to take care of bleeding your hydraulics, contact Kappa Engineering for expert products and advice.

Why bleed a hydraulic cylinder?

When air enters a hydraulic cylinder via the hydraulic pump, it will contaminate the hydraulic fluid which can cause cavitation or aeration. 

Cavitation is a phenomenon which occurs when a rapid change of pressure in a liquid, like hydraulic fluid, leads to the formation of small vapour-filled cavities in areas where the pressure is at its lowest. When compression takes place within the hydraulic cylinder, the bubbles will implode  leading to metal erosion. The erosion will damage components inside the cylinder like metal seals, further contaminating the hydraulic fluid with metal particles that cause even more damage to the moving components and will ultimately lead to the cylinder failing.

This video is a demonstration of Cavitation in a hydraulic system.

Aeration happens when air leaks into the hydraulic system and dissolves into the hydraulic fluid. Commonly, this occurs through the pump seal or pipe fittings. The degradation of the hydraulic fluid accelerates the wear and tear of other components because the oil isn’t providing the right level of lubrication. This causes overheating in the hydraulic pump and damages the seals. When your hydraulic system experiences overheating, dieseling can occur when the hydraulic fluid mixes with the air. The end result can be an explosion when compression takes place in the hydraulic cylinder.

Bleeding a hydraulic cylinder will let the air that has not yet mixed with the hydraulic fluid to escape. When dealing with hydraulic fluid that already has air mixed in it this should be done carefully. The normal amount of mixed air in hydraulic fluid is about 10%. Too much air mixed with the fluid, however, would appear as froth or foam. This can be dealt with by raising the temperature of the fluid until the air escapes and then passes out of the fluid through a gauze or screen designed to remove air from fluid.

This video shows the result of a hydraulic cylinder not being bled properly.

When to bleed your hydraulic cylinder


If you hear a banging or knocking noise in the hydraulic system this is a good indicator that there is air trapped in the pump or the hydraulic cylinder. The noise you are hearing is created by the air compressing and decompressing. In the event of any such noises, you should inspect the hydraulic cylinder and pump. Make sure to check the seals and inspect the hydraulic fluid for signs of degradation.

This video demonstrates the noise you will hear indicating you need to bleed your hydraulic system.

Before installations

Any single-acting hydraulic cylinder needs to be bled before installation. Air trapped in a system with single acting-cylinders will act as a gas shock absorber. For this reason these cylinders have a breather valve at the top to let air escape. When a new hydraulic cylinder is being tested, you need to check for trapped air pockets as this can cause the seals to blow-out of the cylinder housing.

How to bleed your cylinder

Different hydraulic cylinders will use different methods to bleed the cylinder. Below are some general tips to keep in mind, no matter the type of cylinder you need to bleed. These tips focus on bleeding the entire hydraulic system of trapped air.

  • Gather all the supplies and tools you would need to disassemble a hydraulic system. Make sure you have clean hydraulic fluid and clean empty bottles.
  • Work on an even surface as this makes bleeding the air easier.
  • Remove the parts in your way that would make accessing the hydraulic lines possible.
  • Bleed the lines farthest away from the hydraulic pump first.
  • The lines closest to the pump should be bled last and bleed only one line at a time.
  • Make sure to keep the hydraulic fluid reservoir filled. If it runs dry more air will fill the hydraulic system and you will have to restart the process from the beginning.

This video demonstrates how to bleed a hydraulic system.

How to bleed a single-acting hydraulic cylinder

Move the air to the top of your cylinder to make the release of air more effective. Before you begin the bleeding process, make sure you have enough space to extend the hydraulic cylinder fully to facilitate the operation.

  • Extend the cylinder fully and leave it in this position for a few minutes to allow the air to rise naturally. The amount of air in the cylinder would determine how long this will take.  
  • Partially close the cylinder when all the air has raised to the top of the cylinder. 
  • Open the bleed valve at the top for the air to escape. Leave it open until the hydraulic fluid from the valve flows freely. This indicates all the air has been bled out. 
  • If the fluid is foamy, pass it through some gauze or alternatively, refill the system with new hydraulic fluid.


Bleeding your hydraulic cylinders can maintain the useful life and operating efficiency of your machines. Our world exists as it is today, in part, because of the marvel that is the hydraulic cylinder. To use this engineering marvel to its fullest capabilities you need to make sure your cylinders are properly maintained. If you need any help with repairs or maintenance of your hydraulic cylinders contact us today.

If you need 250 Series hydraulic cylinders, 150 Series cylinders or Tipper Series cylinders, click here to order them from Kappa Engineering. We manufacture hydraulic cylinders with any mounting type or port position you require. We manufacture every component of the cylinder in-house and we design and build custom hydraulics for specialised applications.

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