By Dovydas Kersys · Chapter 28 of 40 in Hydraulic Troubleshooting.
Before you begin: Hydraulic equipment can contain lethal stored energy. Only trained and authorized personnel should perform hydraulic work. Isolate energy, support loads, verify the pressure state, use pressure-rated instruments and follow site-specific lockout/tagout procedures. The diagrams are simplified training illustrations; use the actual machine schematic and manufacturer instructions.
Drift does not automatically mean the piston seals are bad. Oil can leak through the directional valve, through load-holding valves, across cylinder seals, or through external plumbing. Gravity and thermal expansion can also move a cylinder.
In practice, the fastest diagnosis comes from deciding what the component is supposed to control and then measuring the variable on both sides of that component. Pressure tells you about load and restriction; flow tells you about speed; temperature tells you where power is being lost; leakage tells you whether clearances or seals are allowing oil to bypass the intended path.
Figure 28. Simplified diagnostic view for 28. Cylinder Drift, Creep and Internal Leakage.
What the technician may observe
- vertical cylinder slowly lowers
- clamp loses force while idle
- position changes as oil warms
- drift rate changes when hoses are isolated
- both ports show trapped pressure
Diagnostic sequence
- Secure the load independently before isolation tests. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Measure both port pressures during drift. Record the result before changing the next variable so the test actually narrows the fault.
- Determine whether valve leakage can feed or drain the cylinder. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Use approved port-blocking or load-holding tests to isolate the actuator. Record the result before changing the next variable so the test actually narrows the fault.
- Check external leaks and hose expansion. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Consider thermal expansion of trapped oil in tightly blocked circuits. Record the result before changing the next variable so the test actually narrows the fault.
How to interpret the measurements
Begin by asking whether the symptom affects the whole machine or only this function. If other functions sharing the same supply remain normal, a common pump or reservoir fault becomes less likely and the local valve, actuator, plumbing and mechanical load deserve more attention.
Next, compare pressure upstream and downstream while oil is actually flowing. A large pressure difference across a component means that component or passage is consuming hydraulic energy. If flow is low everywhere, investigate pump delivery, inlet conditions, speed and major internal leakage before blaming a local valve.
Finally, repeat the test at the condition that creates the complaint: hot oil, full load, a particular direction, low speed or rapid cycling. Hydraulic clearances, viscosity, pilot pressures and valve forces change with operating condition, so a static test may look normal while the production fault remains.
Corrective actions after the cause is proven
- Repair the component proven to leak.
- Use proper counterbalance or pilot-check valves where load holding is required.
- Avoid unsafe hose-cracking tests.
- Record acceptable drift specification after repair.
Common mistakes
- Adjusting the relief or compensator before recording the original setting and proving that the setting is wrong.
- Replacing a component because it is hot, even though the heat may be arriving from an upstream restriction or continuous bypass flow.
- Calling a problem “low pressure” when the actual complaint is low flow or excessive mechanical load.
- Testing only with cold oil when the production fault appears after the system reaches operating temperature.
Field rule: For 28. cylinder drift, creep and internal leakage, do not replace a component until one measurement or isolation test shows how it fails to perform its intended function.
Symptom-to-next-test table
| Observed condition | First useful test |
|---|---|
| Vertical cylinder slowly lowers | Measure delivered flow and work-port pressure during motion. |
| Clamp loses force while idle | Measure the controlled variable before and after the suspected component. |
| Position changes as oil warms | Measure the controlled variable before and after the suspected component. |
| Drift rate changes when hoses are isolated | Secure the load, then isolate valve leakage from actuator leakage. |
Field practice
Before leaving the machine, write down three numbers that would let another technician judge whether the repair truly restored normal operation. Good choices are usually operating temperature, a pressure at a defined machine state, and either flow or cycle time. This creates the baseline that makes the next fault faster to diagnose.
Continue the series
- Previous chapter: Slow or Weak Hydraulic Cylinders
- Next chapter: Hydraulic Cylinder Seals, Rod Damage and Side Loading
- Browse all Hydraulic Troubleshooting chapters
Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.