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.
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.