By Dovydas Kersys · Chapter 10 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.
Reservoir design supports deaeration, cooling, settling and pump suction. Low level, clogged breathers, poor return placement or a vortex at the suction can introduce air and create symptoms that look like pump failure.
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 10. Simplified diagnostic view for 10. Reservoir Problems, Air Ingress and Low Fluid Level.
What the technician may observe
- foamy oil in sight glass
- erratic or spongy actuator motion
- pump noise changes as cylinder extends
- oil level drops below suction connection
- tank pulls vacuum as oil level changes
Diagnostic sequence
- Check level with cylinders in the machine-defined position. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect the breather and filler cap for blockage or damage. Record the result before changing the next variable so the test actually narrows the fault.
- Look for a suction vortex or return jet aimed at the pump inlet. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Inspect suction hoses and fittings for air leaks that may not leak oil outward. Record the result before changing the next variable so the test actually narrows the fault.
- Check whether return oil has enough residence time to release air. Compare the reading with a known-good baseline or the machine specification rather than relying on a generic value.
- Confirm reservoir baffles and internal plumbing after repairs. 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
- Correct fluid level with the specified fluid.
- Replace clogged breathers and damaged suction seals.
- Repair cracked suction hoses.
- Correct return plumbing that entrains air.
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 10. reservoir problems, air ingress and low fluid level, 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 |
|---|---|
| Foamy oil in sight glass | Check inlet condition, air ingress, fluid condition and pump flow. |
| Erratic or spongy actuator motion | Measure the controlled variable before and after the suspected component. |
| Pump noise changes as cylinder extends | Check inlet condition, air ingress, fluid condition and pump flow. |
| Oil level drops below suction connection | 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: Hydraulic Temperature as a Troubleshooting Signal
- Next chapter: Hydraulic Viscosity, Wrong Oil and Cold-Start Problems
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Adapted from Hydraulic Troubleshooting for Maintenance Technicians, by Dovydas Kersys. © 2026 Dovydas Kersys. Featured cover image is an AI-generated editorial illustration.