Hydraulic Hose Leaking: Causes and Solutions
Reading time: approx. 5 minutes

A leak is never normal

A leaking hydraulic hose or fitting is not an unavoidable fact of life. In most cases there is an identifiable cause — and therefore a solution. The problem is that a leak is often fixed by simply replacing the hose, without investigating why it leaked in the first place. After which the new hose does exactly the same thing a few weeks or months later.

This article covers the most common causes of leakage at the hose and fitting, and what you can do about it.

Leakage at the fitting: the most common cause

The vast majority of leaks on hydraulic hoses occur not in the hose itself, but at the fitting — the point of connection between the hose and the installation.

Wrong thread type

BSP, metric, JIC, ORFS, NPT: each thread type has its own profile, its own pitch and its own sealing method. A BSP male thread in a metric female port may appear to engage at first glance, but will structurally never seal properly. The same applies in reverse.

This is one of the most underestimated causes of leakage. The fitting seems to fit, gets tightened, and leaks a few hours or days later. The solution is not to tighten it further — that damages the thread — but to use the correct fitting.

Not sure which thread type your installation uses? Refer to our article on the most common thread types in hydraulics.

Under-tightened or over-tightened

Under-tightening is obvious: the connection doesn't seal. But over-tightening is at least as problematic. With copper sealing rings (DKOS/metric) or O-rings (ORFS), over-tightening can damage or deform the seal. With tapered thread connections (BSP taper, NPT), it can crack the fitting.

Every fitting has a recommended torque value. In practice this is rarely followed, but when leaks keep recurring at the same point it's worth checking.

Damaged or missing seal

Copper sealing rings lose their deformation capacity after repeated use. O-rings can dry out, crack or deform due to chemical attack from the fluid. A fitting that looks visually perfect can still leak because of a seal that needs replacing.

When reassembling a connection: always replace the seal. A used copper ring or O-ring is not a reliable seal.

Damaged or incorrect seat

In fittings that seal via metal-to-metal contact (JIC 37°, DIN taper), the quality of the seat is critical. Scratches, pitting or contamination on the cone cause leakage, even if the fitting is correctly tightened. This type of damage results from dirt, repeated assembly and disassembly, or the use of fittings with mismatched hardness.

A damaged seat cannot be fixed by tightening harder. The fitting or the connecting block must be replaced.

Leakage at the crimp joint

In crimped hose assemblies, the connection between hose and fitting lies in the crimp joint. Leakage at that point almost always has one of the following causes.

Wrong combination of hose, ferrule and fitting

Hoses, ferrules and fittings are matched to each other by manufacturer and product line. A ferrule from brand A cannot simply be combined with a fitting from brand B, even if they share the same nominal size. Crimp diameters, ferrule geometry and hose constructions are not universal.

In practice, combinations that "look right" are used without being certified. This can work fine for years — and then fail under pressure or vibration.

Incorrect crimp setting

Too little crimp: the fitting holds in place, but the seal is insufficient. The fitting can then blow off when the system is pressurised. Too much crimp damages the hose construction or the fitting.

Every combination of hose, ferrule and fitting has a specific crimp diameter, which you can find in our catalogue. A correct crimp setting starts with the right data, not with experience or gut feeling.

Fitting not fully inserted

It sounds basic, but it happens. If the hose is not pushed all the way to the stop of the fitting before crimping, the crimp connection is too short and insufficiently strong. A visual check before crimping — via the inspection hole in the ferrule or by measurement — is never a wasted step.

Leakage along the hose wall

If the hose itself is leaking — not at the fitting, but somewhere along its length — this indicates damage to the hose construction.

Mechanical damage

Cuts, abrasion and punctures from sharp edges, bolts or contact with moving parts are a frequent cause. The outer cover protects the reinforcement and inner tube: once cut through or heavily worn, moisture and dirt penetrate to the reinforcement and corrosion begins.

A hose with visible damage to the outer cover that isn't leaking yet will leak eventually. Replace it preventively.

Excessive bending or kinking

Every hose has a minimum bend radius. If that radius is exceeded, the reinforcement layers buckle. At a kink, pressure concentrates at a single point. The hose leaks or bursts there — sometimes after a period of time, sometimes immediately.

Kinks are recognisable as a sudden narrowing or fold in the hose. A kinked section cannot be repaired: the hose must be replaced, and the routing of the installation must be adjusted so that the minimum bend radius is respected.

Ageing and chemical attack

Rubber ages. Under the influence of heat, UV radiation, ozone and the chemical action of the fluid, the rubber layers become more brittle and lose their elasticity. Surface cracking on the outer cover is an early sign of ageing.

Chemical attack on the inner tube — from an incompatible fluid or an additive — is less visible. The hose looks normal on the outside, but the inner tube is degraded. Symptoms include softening of the hose wall, swelling or an unusual smell in the oil flowing out.

When leaks keep recurring on the same installation, always check whether the fluid in use is still compatible with the hose — especially after a maintenance change of oil type or the addition of additives.

Pressure spikes and fatigue

Hydraulic systems generate pressure spikes: short, intense pressure surges when a pump starts or stops, a valve closes quickly, or a cylinder reaches end of stroke. A single spike rarely breaks a hose immediately. But repeated spikes above the recommended working pressure fatigue the reinforcement.

A hose that fails due to pressure fatigue often shows no external damage: it bursts suddenly, sometimes along a seam in the reinforcement. If hoses on a given installation consistently fail prematurely without an obvious mechanical cause, pressure spikes are a serious suspect.

The solution is not simply to select a heavier-duty hose: the height and frequency of the spikes need to be established first. Sometimes a pressure regulator or a surge damper is sufficient.

What to do with a recurring leak?

Don't just replace the same hose in the same way. Work through these questions first:

Where exactly is the leak — fitting, crimp joint or hose wall?
Is the combination of hose, ferrule and fitting certified to work together?
Were the crimp setting and torque value correctly applied?
Is the thread type correct and is the seal in good condition?
Is the minimum bend radius respected in the current installation layout?
Is the fluid compatible with the hose?
Are there any indications of pressure spikes?

A recurring leak is sending a signal. It's worth taking that signal seriously.

Need help?

At Van de Calseyde Hydraulics, we help you identify the cause of a recurring leak. Bring in the defective hose or describe the installation, and we'll work through it together.