Hydraulic Cylinder Internal Leakage – The Invisible Problem

When people think about hydraulic failures, they picture oil on the floor, blown hoses and obvious external leaks that stop production in its tracks.

But some of the most expensive hydraulic failures never leave a drop of oil on the ground. They happen silently inside your cylinders – slowly robbing your machines of power, precision and reliability.

That’s the reality of hydraulic cylinder internal leakage: the invisible problem that can quietly erode productivity for months before anyone realises what’s going on.

In this guide, we’ll unpack what internal leakage really is, why it’s so hard to spot, and how you can diagnose, prevent and fix it before it becomes an emergency.

What is hydraulic cylinder internal leakage?

In simple terms, internal leakage happens when pressurised oil bypasses the piston seal (or occasionally the rod seal) and flows from the high pressure side of the cylinder to the low pressure side inside the cylinder barrel.

Instead of that oil doing useful work (lifting a boom, pushing a ram, clamping a load), it sneaks around the piston and returns to tank or equalises pressure on both sides of the piston.

The result is a cascade of problems:

Loss of force – the cylinder can’t hit its rated push or pull
Load drift – booms, tables or jibs slowly sink under load
Slower cycles – machines feel “sluggish” and unresponsive
Heat build-up – energy is turned into heat instead of useful work
Higher energy costs – the pump has to work harder for less output

Externally, the system may look fine. No visible oil. No blown hoses. Yet cycle after cycle, you’re paying for lost efficiency and accelerated wear.

Why internal leakage is so hard to detect

External leaks are easy:

Oil on the floor
Oil on the rod
Drips from fittings

Someone sees it, calls maintenance, and the problem gets fixed.

Internal leaks are the opposite:

No visible oil escape
Symptoms build up slowly over weeks or months
Operators adapt to the “new normal” of slower cycles
It’s often misdiagnosed as valve problems or “just old equipment”

By the time internal leakage is obvious, the damage is usually well advanced. Temperatures are up, seals are cooked, bores are worn, rods are marked, and the repair bill is significantly higher than it needed to be.

Suspect internal hydraulic cylinder leakage? Talk to Berendsen’s expert repair team for practical advice and fast, reliable repairs.

The physics: why small wear becomes a big problem

From a physics point of view, internal leakage is simply oil flowing through a tiny gap between piston and bore. The important bit is this:

Leakage increases with the cube of the clearance.

If the clearance between piston and bore doubles due to seal wear or bore damage, leakage doesn’t just double – it can increase up to eight times.

That’s why internal leakage tends to feel “okay, okay, okay… and then suddenly terrible”. The last part of seal life is where the leakage curve ramps up sharply.

So a slightly worn seal that was only leaking a fraction of a millilitre per second can suddenly become a major leak, dragging down performance and generating a lot of heat in a very short time.

Common causes of internal leakage in hydraulic cylinders

Internal leakage is rarely just “old age”. It’s usually the result of one or more of these underlying issues.

1. Piston seal wear and degradation

This is the most common cause.

Over time, piston seals:

Wear down from constant sliding contact
Harden or soften with high temperature
Lose elasticity and can no longer maintain contact with the bore
Suffer from chemical attack from degraded or contaminated oil

Temperature is a huge factor. Running just 10–20°C hotter than ideal can cut seal life almost in half and significantly increase internal leakage.

2. Cylinder bore damage

The bore only needs minor damage to cause leakage:

Scoring and scratching from contamination or misalignment
Out-of-roundness (ovality) from pressure and mechanical stress
Ballooning of the barrel when pressure repeatedly exceeds design limits

Even a few hundredths of a millimetre of bore distortion can create bypass paths that piston seals simply cannot bridge, especially under high pressure.

3. Rod damage and misalignment

The piston rod is the “guide rail” for the piston. When it’s not right, everything downstream suffers.

Common issues include:

Bent rods from side loads or impacts
Damaged or corroded chrome that chews out seals
Poor surface finish that spikes friction and heat

A bent rod will tend to push the piston against one side of the bore, leading to uneven seal wear and scoring on one side of the barrel. That combination is a perfect recipe for internal leakage.

4. System-level problems

Sometimes the cylinder is just the victim:

Worn directional valves can leak internally, upsetting pressure balance
Poor control of fluid temperature accelerates seal damage
Dirty oil drives abrasion and scoring inside the cylinder
Incorrect mounting and alignment over-stress the cylinder mechanically

If those systemic issues aren’t addressed, a rebuilt cylinder can quickly end up right back where it started.

Early warning signs: how internal leakage shows up in the real world

Because you can’t see internal leakage, you have to feel and measure it. Here are the tell-tale signs to watch for.

1. Cylinder drift under load

One of the clearest indicators is drift:

A boom slowly lowers with the valve in neutral
A clamp loses pressure and relaxes
A raised platform sinks over time

Even a few millimetres of movement over a few minutes can be the first sign that piston seals are no longer holding pressure properly.

2. Slower extension and retraction speeds

When internal leakage increases, part of the pump flow bypasses the piston inside the cylinder instead of moving the load.

To the operator, that looks and feels like:

Slower cycles for the same control inputs
The system needing more revs or more time to achieve the same movement
A general feeling that the machine is “lazy” or “bogged down”

This is often blamed on the pump or the valve, but a leaking cylinder can easily be the true culprit.

3. Inability to hold load position

If a cylinder can’t hold a heavy load steady with the valve in neutral, you’re almost certainly dealing with:

Piston seal leakage
Valve spool leakage
Or a combination of both

From a safety point of view, this is critical. Uncontrolled or unexpected movement of heavy equipment is a serious risk to people and assets.

4. Rising oil temperature

Internal leakage generates heat. A system that:

Used to run happily around 65–75°C
Now regularly climbs into the 80–90°C range

…is waving a red flag.

At that point you’re not just dealing with leakage. The high temperature is now actively attacking seals, oil and other components, making the problem worse.

Simple ways to test for internal leakage

You don’t always need exotic test rigs to get a handle on what’s going on. A few structured checks can tell you a lot.

1. Drift test

A basic field test you can do with simple tools:

Run the machine until it’s at normal operating temperature.
Extend or retract the cylinder under a representative load.
Put the control valve in neutral.
Mark the rod position or measure from a fixed reference point.
Wait 5 minutes.
Measure movement.

Small movement (a millimetre or two) can be acceptable in some applications. Anything more than that, especially on critical lifting or holding cylinders, is a strong sign that internal leakage is rising.

2. Bypass (flow) test

Where possible, a more precise method is to:

Pressurise one side of the cylinder
Temporarily route the opposite port to a measuring container or flow meter
Measure how much oil bypasses the piston over a set period

This gives you an actual leakage rate in ml/min or L/min and makes it easier to decide whether a repair can be scheduled or needs to be treated as urgent.

How to prevent internal leakage before it starts

The good news: you can dramatically cut the incidence of internal leakage with a few disciplined practices.

1. Keep hydraulic fluid clean

Contamination is one of the biggest enemies of cylinder seals and bores. To prevent internal leakage:

Use properly sized, high-efficiency filtration
Monitor oil cleanliness against ISO 4406 targets
Keep dirt out during top-ups and maintenance
Change filters based on condition, not just hours

Every particle that gets between the piston seal and bore is a potential scratch, groove or wear point that opens the door to bypass.

2. Control temperature

Aim to run your system in a sensible temperature window.

Too cold and oil is thick, energy-hungry and slow
Too hot and seals, oil and coatings degrade rapidly

As a rough guide, cylinder seals live longest when the oil runs in the 65–75°C range. Run consistently in the 85–95°C band and you’ll pay for it in seal life and leakage.

If you’re routinely seeing higher temperatures:

Check for internal leakage in cylinders and valves
Review cooling capacity and flow
Check for incorrect relief settings or constant overloading

3. Protect rods and keep cylinders aligned

Good cylinder mechanics make an enormous difference:

Avoid side loading cylinders whenever possible
Make sure mounts and pins are correctly sized and maintained
Inspect rods regularly for damage or corrosion
Fit rod boots or protection where environment is harsh

A straight, smooth, well protected rod is one of your best defences against internal leakage.

4. Stick to a preventive maintenance plan

A simple structured plan might include:

Daily: Visual check, listen for unusual noises, watch for drift
Weekly: Check operation under load, note any slow or jerky motion
Monthly: Focused inspection of key cylinders, rods and hoses
Annually: Full inspection and, where justified, scheduled cylinder rebuilds

The goal is to catch seals and components at the early leakage stage rather than waiting for catastrophic failure.

Repair or replace? Making the right call

Once you’ve identified a cylinder with internal leakage, the next question is what to do with it.

Repair makes sense when:

Damage is mainly limited to seals and minor bore/rod wear
The rod can be re-chromed and the bore can be honed
The cylinder is otherwise well designed for the application
Repair cost is clearly lower than buying new

A proper rebuild typically involves:

Full strip and clean
Detailed inspection and measurement of bore, rod and piston
Honing and reconditioning where needed
Rod re-chrome or replacement if damaged
Quality seal kit replacement
Pressure and leakage testing before return to service

Replacement becomes the better option when:

The bore is badly ballooned or out-of-round
The rod is badly bent or heavily corroded
The cylinder has already been repaired multiple times
Spares are obsolete or excessively expensive
The application has changed and a better-spec cylinder is justified

The hidden cost in both scenarios is downtime. Catching internal leakage early gives you the luxury of planning repairs around production, instead of scrambling when a cylinder finally gives up.

FAQs: hydraulic cylinder internal leakage

Is a small amount of internal leakage normal?

Yes. No seal is perfect. Brand new cylinders will have a tiny amount of bypass, but it’s usually so low it has no practical effect on performance. What matters is trend – how fast that leakage increases over time.

Can I fix internal leakage by using thicker oil?

Not really. Changing to a higher viscosity oil might slightly reduce leakage at the margins, but it doesn’t address the underlying problem: worn seals, damaged bores or rods, heat or contamination. In some cases it can actually create new issues with cold start performance and energy use.

Does internal leakage always mean the cylinder is at fault?

No. Worn directional control valves and other components can create similar symptoms (drift, loss of holding, pressure issues). That’s why structured testing is important – to separate valve leakage from true cylinder leakage.

How much does internal leakage really cost?

The obvious cost is the repair itself. The bigger hidden costs are:

Longer cycle times
Higher fuel or energy consumption
Overheated oil shortening the life of everything it touches
Emergency breakdowns and unplanned downtime

Catching and fixing internal leakage early almost always works out cheaper than running a cylinder to the point of failure.

The invisible problem you don’t have to live with

Hydraulic cylinder internal leakage might be invisible, but it isn’t mysterious.

It’s the predictable result of wear, heat, contamination, misalignment and time. And with the right monitoring, maintenance and repair strategy, you can:

Spot it early
Control it before it snowballs
Extend cylinder life
Protect operators and equipment
Reduce overall maintenance spend

If your machines feel slower than they used to, struggle to hold load, or are running hotter than normal, don’t just turn the radio up and live with it. Those are classic signs that internal leakage could be quietly eating away at your hydraulic system from the inside.

Treat internal leakage as a measurable, manageable reliability problem – not a vague “machine getting old” issue – and you’ll be well ahead of the curve.