What Internal Leakage Means
Internal leakage is hydraulic fluid moving across a sealing boundary inside the cylinder instead of remaining separated between working chambers. In a double-acting cylinder, the most familiar path is past the piston sealing system, but internal static seals, damaged components or cracks can also create bypass paths.
Because the oil remains inside the hydraulic circuit, there may be no visible external leak.
Typical Symptoms
Enerpac notes that piston-seal leakage can cause sluggish movement or settling under load. Parker troubleshooting also associates internal bypass with failure to hold load and reduced actuator performance.
Common symptoms therefore include drift, weak holding, reduced effective force, slower motion and heating. None of these symptoms, by itself, proves that the cylinder is the source.
Separate Cylinder Bypass from Valve Leakage
A directional control valve has internal clearances and can leak between ports. Load-holding valves and other circuit elements can also provide internal leakage paths.
For this reason, a drifting cylinder should not automatically be rebuilt. The leakage path must be isolated using the equipment and cylinder manufacturer's approved diagnostic method.
Piston Seal and Bore Damage
Worn piston seals are a common source of bypass, but the seal is not the only surface that matters. A scored or damaged bore can prevent a new seal from controlling leakage, and contamination can damage both the seal and cylinder surface.
Inspect the piston, bore and associated static seals if teardown is justified.
Heat as Supporting Evidence
Fluid throttling through a leakage path converts hydraulic power to heat. Parker notes that internal leakage past a seal can contribute to rapid temperature increase.
Heat is supporting evidence rather than proof because restrictions, friction and other hydraulic losses can produce similar temperature rise.
Leakage Limits Depend on Construction
Do not apply one generic allowable leakage value to every industrial cylinder. Parker distinguishes between piston-ring constructions, which can have a specified leakage allowance, and soft-seal designs that are expected to have very low static bypass.
Use the manufacturer, series and seal construction as the acceptance basis.
Repair Decision
When internal cylinder leakage is verified, inspect the full sealing system and its failure mechanism. Replace seals only if the bore, piston and other surfaces meet requirements. If internal damage, cracking or geometry problems cannot be restored reliably, replacement review is more appropriate.