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Troubleshooting Article Archive:  March 2006      
That Sinking Feeling of Hydraulic Sinkage
Operate a tow truck long enough and eventually you get to know that sinking feeling.  You look in the rearview mirror as you drive and the towed vehicle is gradually but inexorably sinking down towards the highway.  Regardless of whether oil is actually leaking out on the ground, hydraulic sinkage, sometimes called "drift," can occur.

Hydraulics work as a system.  Replacing one component will not necessarily solve the problem of sinkage.  For example, both the valve body and the cylinder could be worn out inside and bypassing internally.  In such a case, rebuilding or replacing the cylinder is certainly in order, but rebuilding or replacing only the cylinder will result in zero improvement in the sinkage problem:  the oil will simply bypass through the valve body and the cylinder will sink as before.

Hydraulic Arrangements

I describe the two most common hydraulic arrangements on tow trucks as valve body to cylinder, and valve body to cartridge valve to cylinder.  In the first case, where there is only a valve body and a cylinder, both components need to be in 100 percent condition to hold a load without sinkage.  In the second case, where the manufacturer has installed a cartridge valve between the valve body and the cylinder, the cartridge valve works to eliminate the valve body, for purposes of sinkage, from the system so that only the cylinder and the cartridge valve need to be in 100 percent condition to hold a load.

Note that in either of the above scenarios the cylinder must be good.  Inescapably, the cylinder must be in 100 percent condition or the sinkage will continue, regardless of what other components may also be worn out.  Thus, in logicians' language, making sure the cylinder's good is necessary, but not sufficient.

Also note that on older, high-hours equipment, it is quite likely that all of the components of the system are worn out and bypassing internally, since the valve body, cartridge valve and cylinder are all of the same vintage and all have about the same hours of operation on them.  And they all contain similar types of o-ring and sealing materials.


When troubleshooting hydraulic sinkage, first repair the cylinder since it's the one component that must be 100 percent in all cases.  Then load the equipment and look for sinkage.  If it still sinks, and there's a cartridge valve, fix the cartridge valve.  A 100 percent cartridge valve and a 100 percent cylinder will positively eliminate sinkage even if the valve body is bypassing internally.  If there is no cartridge valve, fix the valve body.  A 100 percent valve body and a 100 percent cylinder will positively prevent sinkage.