Understanding Hydraulic Cylinder Internal Leaks

First, it’s important to understand what an internal leak in a hydraulic cylinder is. Unlike external leaks, which are visible as oil escaping from the cylinder, internal leaks occur when fluid bypasses internal components like seals or valves without leaving the cylinder. These leaks often occur between the piston and the cylinder bore or across the piston’s seals.

Symptoms of Internal Leakage

Several symptoms can indicate internal leakage in a hydraulic cylinder:
hydraulic cylinder

  • Reduced Performance: The most common symptom is a decrease in the cylinder’s performance or force output.
  • Slower Operation: The cylinder may extend or retract more slowly than usual.
  • Erratic Movement: The cylinder may move erratically or seem less smooth in its operation.
  • Overheating: Excessive internal leakage can cause the hydraulic fluid to overheat.
  • Increased Fluid Consumption: If you’re having to top off the hydraulic fluid more frequently, it could indicate leakage.

Tools and Preparation

Before beginning, ensure you have the necessary tools. These might include:

  • Pressure gauge
  • Flow meter
  • Hydraulic system schematic
  • Safety gear (gloves, goggles)

Also, make sure the system is turned off and pressure is released before you start working on it.

Checking for Internal Leaks

  1. Visual Inspection: Start with a visual inspection of the cylinder for any signs of external damage that might suggest internal problems.
  2. Check for External Leaks: Before diagnosing internal leaks, confirm there are no external leaks. Any fluid on the outside of the cylinder or along the hydraulic lines can be a sign of an external leak.
  3. Pressure Testing:
    • Attach a pressure gauge to the system.
    • Activate the hydraulic system and monitor the pressure.
    • A drop in pressure without a corresponding cylinder movement can indicate an internal leak.
  4. Perform a Cylinder Bypass Test:
    • Extend the cylinder fully and mark the position of the rod.
    • Shut off the system and monitor if the rod retracts or moves from the marked position over time.
    • Movement without system pressure can indicate fluid is bypassing the piston seals internally.
  5. Flow Meter Test:
    • Install a flow meter in the hydraulic circuit.
    • Measure the flow rate when the cylinder is in operation and when it’s at rest.
    • Excessive flow when the cylinder should be stationary can indicate an internal leak.
  6. Temperature Testing:
    • Using a temperature gun, check the temperature of the fluid.
    • Overheating can be a sign of internal leakage due to the extra work the system has to do to maintain pressure.
  7. Use of Hydraulic Schematics:
    • Consult the hydraulic schematic of your system.
    • It can help you understand the potential points where internal leakage could occur.
  8. Inspection of Cylinder Components:
    • If you suspect internal leakage, the cylinder may need to be disassembled for inspection.
    • Check the piston seals and the cylinder bore for wear or damage.
    • Look for scoring or scratches on the piston rod, which can indicate seal wear.

hydraulic cylinder

After Identifying a Leak

Once you’ve identified an internal leak, it’s important to address it promptly:

  1. Replace Worn Components: Any worn seals or damaged parts should be replaced. Always use parts that meet the original equipment manufacturer’s (OEM) specifications.
  2. Professional Repair: If the leak is due to a more complex issue or if you’re unsure about making repairs yourself, consult a professional.
  3. Reassemble and Test: After repairs, reassemble the cylinder and test it to ensure the leak has been fixed. Monitor for any pressure drops or performance issues.
  4. Regular Maintenance: Implement a regular maintenance schedule to check for leaks and ensure the longevity of your hydraulic system.

Preventing Future Leaks

Prevention is always better than cure. To prevent future internal leaks:

  • Regular Maintenance: Regularly check and replace worn seals and components.
  • Use Quality Hydraulic Fluid: Ensure you use the correct type and quality of hydraulic fluid.
  • Proper System Operation: Train operators to use the hydraulic system correctly to avoid overstressing components.
  • Regular Inspections: Frequent inspections can help catch issues before they lead to internal leaks.


Internal leaks in hydraulic cylinders can be challenging to detect but are critical to the efficient operation of your hydraulic system. By using the right tools and techniques, such as pressure testing, bypass testing, and visual inspections, you can accurately diagnose and address these leaks. Regular maintenance and preventive measures are key to minimizing the risk of internal leakage and extending the life of your hydraulic equipment.