September 21, 2017
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Flushing the Hydraulic System

Replacing hydraulic fluid is simply draining old fluid and adding new fluid.  Flushing the entire system, on the other hand, is much more thorough as it flushes out both contaminants and old fluid. In an ideal world, a hydraulic system would never need to be flushed.  However, machines operate in the real world.  Since there are situations in which hydraulic fluid would need to be flushed, we will discuss in this Shop Talk Blog post the why and how of flushing the hydraulic system on your heavy equipment.


What Is a Hydraulic Flush?

A hydraulic flush purges the system of degraded fluid, sludge, and contaminants (including water, air, and solid particles). A flush should not be necessary for a machine that has been regularly maintained and has not experienced any major failures or contamination. In such cases, draining and adding fresh hydraulic fluid according to manufacturer recommendations is sufficient.

When Should a Hydraulic System be Flushed?

There are numerous situations in which a full hydraulic system flush is recommended. If your equipment is new or has recently been overhauled, it would be wise to flush your system to purge out any built-in contamination. The same goes for equipment that has been sitting a long time or has experienced a major failure recently. This is especially true if that failure involved a filter or contamination.  You should also flush your system if your hydraulic fluid has formed sludge. Another potential reason for a hydraulic flush is to prepare the machine for a new type of fluid. Because certain additives may not be compatible, combining different types of hydraulic fluid is not a good idea.

How Is a Hydraulic Flush Performed?

There are many different techniques for performing a system flush, but one that many experts recommend is the “double oil and filter change.” Here is an excellent summary from Hydraulics & Pneumatics:

A double oil and filter change involves an initial oil drain and filter change, which expels a large percentage of contaminants and degraded fluid. The system is then filled to the minimum level required and the fluid circulated until operating temperature is reached and the fluid has been turned over at least five times. The oil [hydraulic fluid] is then drained and the filters changed a second time.


Your should drain the system only after the existing fluid has reached a normal operating temperature. Draining includes the reservoir as well as accumulators, cylinders, filter housings, lines, and any other areas where hydraulic fluid accumulates. Afterward, clean the reservoir with a lint-free rag to remove all sludge and deposits.
Other methods of flushing include power flushes or high velocity flushes (which require a flushing rig) and chemical power flushes. However, the double oil and filter change seems to be the best option for those without access to a power flushing rig.


Contaminants and degraded fluid negatively impact the performance of your system. If you suspect that your hydraulic system has been compromised through contamination or system failure, perform a double oil and filter flush — especially if your system has recently experienced a catastrophic final drive failure.

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