How to Tell If Your Hydraulic Cylinder Is Leaking Internally: 5 Warning Signs

Internal leaks in hydraulic cylinders are one of the most frustrating problems in hydraulic systems, reducing performance and wasting energy but difficult to diagnose. Unlike external leaks that leave visible signs, internal leaks hide inside the cylinder, slowly degrading system performance until critical failure occurs.

At BSP Hydraulics, we diagnose and repair hydraulic equipment including cylinders across Yorkshire. In this guide, we’ll show you how to identify internal cylinder leakage before it causes expensive downtime.

What Is Internal Hydraulic Cylinder Leakage?

Internal leaks can happen when hydraulic fluid bypasses the piston seal and flows from one side of the piston to the other inside the cylinder. The piston seal is designed to create a complete barrier but when the seal degrades, pressurised fluid finds a way around the piston, creating an internal flow that:

• Reduces cylinder force and speed
• Prevents the cylinder from holding position under load
• Generates excessive heat in the hydraulic system
• Wastes energy as the pump works harder to compensate
• Eventually damages other seals, rods and cylinder components

Sign 1: Cylinder Drift Under Load

This is the most common and reliable indicator of internal leakage.

The Test:

1. Extend your cylinder fully under normal working pressure
2. Apply a moderate load (if possible in your application)
3. Shut off pressure supply to that cylinder port
4. Observe for 2-5 minutes

What to Look For:

Normal behaviour: The cylinder remains fully extended, holding position firmly.

Internal leak behaviour: When there’s a leak, the cylinder can start to slowly drift back over time, even though no external force is pulling it. The rod may move a few millimetres or several centimetres depending on the severity of the leak.

Sign 2: Reduced Cylinder Force

If your cylinder no longer pushes or pulls with its original strength, internal leakage is a likely cause.

How to Identify:

• Loads that were previously easy now strain the cylinder or move slowly
• Cylinder struggles to reach full extension or retraction
• Cycle times increase as the cylinder takes longer to complete movements
• Inconsistent performance- sometimes works normally, other times lacks power

Sign 3: Slow or Sluggish Cylinder Movement

Internal leaks don’t just reduce force, they  can also slow cylinder speed under load.

What You’ll Notice:

• Extension or retraction takes longer than it should
• Slow start to movement, especially when changing direction
• Inconsistent speed- starts fast, then slows mid-stroke
• Asymmetric speed- extends normally but retracts slowly (or vice versa)

Sign 4: Return Line Flow When Cylinder Should Be Static

This test requires access to your return line and is particularly effective for confirming internal leakage.

The Test:

1. Position cylinder mid-stroke
2. Stop all cylinder movement (close directional valve to neutral)
3. Keep system pressurised
4. Watch the return line flow

What to Look For:

Normal behaviour: No flow from the return line- all oil stays in the system.

Internal leak behaviour: Continuous or intermittent flow from the return line back to the tank when the cylinder isn’t supposed to be moving.

Flow rate indicates severity:

• Steady trickle = minor leak
• Constant stream = moderate leak
• Heavy flow = severe leak

Sign 5: Excessive System Heat

Wasted energy converts to heat. If your hydraulic system runs hotter than normal, internal leaks may be contributing.

Why Internal Leaks Generate Heat:

Hydraulic fluid forced through the gap between piston and seal undergoes throttling, converting pressure energy into heat. This leads to your pump working harder to maintain system performance, creating additional heat which can lead to seals degrading quicker. 

Other Heat Sources:

Internal cylinder leaks aren’t the only cause of system heat. Also check relief valves are set correctly, return lines are correctly sized and you’re using the right pump speed.

If heat correlates with cylinder usage- i.e. system heats up when cylinders cycle and cools when static, an internal leak could be the issue.

What Causes Internal Cylinder Leakage?

Understanding causes helps prevent future failures. Internal leakage typically results from:

1. Seal Wear (Most Common)

Age and cycles: Piston seals have a finite lifespan. After thousands of cycles, the compounds harden and crack.

Typical seal life: 3-7 years depending on usage, though some applications wear seals much quicker.

2. Contamination

Particles in hydraulic fluid act like grinding paste between the piston seal and cylinder wall. Even microscopic contamination accelerates wear.

Sources: Worn pump components, external ingestion, degraded seals, poor maintenance

Prevention: Maintain filtration, change filters regularly and keep hydraulic reservoir sealed.

3. Excessive Temperature

Heat degradation means seals lose elasticity, shrink and crack when exposed to high temperatures.

Prevention: Keep operating temperature in range and use high-temp seals for hot environments or improve cooling systems.

4. Pressure Spikes

Sudden pressure surges can momentarily exceed seal design limits, causing immediate damage or accelerated wear.

Repair vs Replace

When you’ve confirmed internal leakage, you’ll need to either get the cylinder repaired or replace it entirely. BSP technicians will be best placed to advise you but here’s a general guide:

Repair Makes Sense When:

• Cylinder barrel and rod are in good condition (no scoring or corrosion)
• It’s only the seals needing replacement
• Cylinder is a standard size readily available for seal kits
• Downtime for repair is acceptable
• Repair cost is less than 50% of the replacement cost

Replace Makes Sense When:

• Cylinder barrel is scored or pitted
• Chrome on the rod is damaged or corroded
• The cylinder is very old (10+ years) or has multiple previous repairs
• Obsolete model where the seal kits are difficult to source 

Preventing Internal Cylinder Leakage

Prevention is cheaper than repair. Follow these best practices:

1. Maintain Clean Hydraulic Fluid

• Change filters per manufacturer schedule (or when pressure drop indicates)
• Conduct oil analysis annually
• Keep the reservoir sealed and clean

2. Control Operating Temperature

• Keep fluid temperature in the correct range
• Add cooling capacity if the system runs hot
• Check for sources of heat generation (restrictions, relief valve bypassing)

3. Use Appropriate Seal Materials

• Verify seal compound matches hydraulic fluid type
• Use high-temperature seals for hot applications
• Consider polyurethane for high-pressure applications
• Use Viton for biodegradable or fire-resistant fluids

4. Regular Maintenance Inspection

• Monthly: Visual inspection for external leaks, unusual noises
• Quarterly: Performance check, does cylinder cycle normally?
• Annually: Measure cycle times, check for drift under load
• Record observations to identify gradual degradation

5. Proper Cylinder Mounting

• Ensure cylinder mounting prevents side loading
• Check alignment- rod should move freely without binding
• Use spherical bearings where misalignment is possible
• Inspect mounts regularly for wear or looseness

If you have any questions about our range of hydraulic pumps feel free to contact us – our team of specialists are here to help.