The hydraulic reservoir is not a sealed box. Its oil level rises and falls as the cylinders and accumulators take and return oil, and the air above the oil has to move in and out to allow that. The tank breather is the component that lets it, and on most machines the breather also carries a filter element so the air entering is cleaned. When the breather element loads up or the breather is painted over or damaged, the tank can no longer breathe freely, and the machine finds another way in: through the filler cap seal, a rod seal, or the weakest gasket on the tank. That route is unfiltered, so the tank draws in dust and moisture, and the particle count rises without any obvious fault. The suction strainer is the second neglected component. It sits at the tank outlet and protects the pump from anything large enough to destroy it, and it is commonly cleaned or checked far less often than the filters that are easy to reach.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Particle counts creeping up with no component wear found | Ingress through an unfiltered path raises the count without producing a mechanical fault, which is why the breather belongs in the investigation for a rising cleanliness trend. |
| Oil weeping from the filler cap or the tank gasket | Positive pressure in the tank pushes oil past the weakest seal. A breather that cannot vent is the usual cause of an oil film around the filler neck. |
| Hydraulic noise at idle or a whine that changes with oil temperature | A restricted suction strainer or a tank under vacuum makes the pump work to draw oil. The noise is the pump telling you it is not being fed properly. |
| Water found in the oil during a sample but no external leak | A machine that pulls in damp air through an unfiltered path condenses water in the tank. Free water in a sample with no visible leak points at the breather and the tank environment. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Loaded or saturated breather element | The element does what it is designed to do until it is full. Once saturated, the pressure drop across it rises and the tank starts to find air elsewhere. |
| Breather blocked by paint, mud or a damaged cap | Common after a repaint or on machines working in mud and dust. The breather may look intact and be completely sealed. |
| Suction strainer partially blocked | Debris, seal fragments or sludge on the strainer restricts flow to the pump, which shows up as noise and reduced performance rather than as a clean failure. |
| Worn filler cap seal or a degraded tank gasket | Once the seal has hardened, the tank breathes through it in both directions, and the air it draws carries dust and moisture straight into the oil. |
Servicing both components
| Item | What the service involves |
|---|---|
| Breather housing and cap | Clean the outside before opening anything. Check the housing is not cracked and the mounting is sound, and confirm nothing is painted over the vent. |
| Breather element | Renew rather than clean. A saturated element does not recover, and washing it disturbs the media. Fit the correct element for the machine. |
| Suction strainer | Remove, inspect and clean only if the specification allows it, then check the mesh for damage and for metal particles. A damaged strainer is replaced. |
| Seals and gaskets disturbed during the job | Renew every seal opened to reach the strainer or the breather. Reusing a hardened tank gasket reopens the unfiltered air path the service was meant to close. |
| Magnet and tank interior | Where the design includes a magnetic plug, check what it has collected. Fine gray paste is normal; chips or flakes are not, and they change the diagnosis. |
How to confirm it
- Clean the area before opening anything
Wash or wipe down the breather, filler cap and the tank top before a tool goes near them. Any dirt sitting on the tank during this job falls in when the components come out, which is the opposite of the intention.
- Inspect the breather housing and cap for blockage rather than wear
Look for paint, mud, or a vent that has been covered. On machines that have been through a repaint, this is often the entire fault. Confirm the housing and the mounting are sound before fitting a new element.
- Renew the breather element on the interval, and sooner in dusty conditions
Treat it as a consumable like an air filter. Machines working in demolition, quarrying or sandy ground load the breather far faster than the manual interval suggests, and the condition of the element is a better guide than the calendar.
- Check filler cap and tank seals while the area is open
Squeeze the cap seal and look for hardening or cracking. This is a low-cost part and one of the two main unfiltered air paths into the tank, so renewing it at the same time is efficient.
- Remove and inspect the suction strainer
Follow the manufacturer's procedure, which on many models requires draining the tank. Clean it only where the specification permits, then inspect the mesh for damage and for metal. Any metal on the strainer means something upstream is failing and the pump is at risk.
- Refill, seal and confirm with a sample
Refill through a filtered arrangement, renew every seal disturbed, and take a sample after the next interval of work. Cleanliness after this service is confirmed by the trend, not by the fact that the service was done.
What to do about it
Renew rather than clean the parts that cannot be restored: the breather element, the cap seal and any gasket opened during the job. Cleaning a saturated element and refitting a hardened seal leaves the tank breathing through the wrong path and the problem returns within the same service interval.
Where the suction strainer has collected metal, stop and investigate before returning the machine to work. The strainer is the last line of defense in front of the pump, and metal on it means the pump is being fed fragments. Identify the source — a failing cylinder, a worn pump, a disintegrating hose liner — and repair it, because a new strainer does not stop the wear that filled the old one.
Add the breather and strainer to the routine service schedule rather than treating them as repairs. Both are inexpensive, both are commonly skipped, and both show up later as a particle count problem or a pump failure that looks unrelated. Breathers, elements, suction strainers, tank seals, gaskets and hydraulic filter elements are supplied by Guangzhou Shenyue Machinery Technology Co., Ltd. from Guangzhou, with 3,000+ part types on the shelf and 100,000+ part numbers available to order, the grade of every line declared. The five brands for which the company holds authorized dealer status are Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan, and the Mitsubishi authorization covers MHI engine parts only. Kobelco is not among them, and Kobelco parts are supplied through original channels or as quality alternatives.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Breathers and breather elements | Genuine; OEM-quality; matched aftermarket element to the same filtration rating. |
| Suction strainers | Genuine; OEM-quality; matched aftermarket strainer to the specified mesh. |
| Tank seals, gaskets and filler caps | Genuine; OEM-quality; specified material for hydraulic oil contact. |
| Hydraulic filter elements | Genuine; OEM-quality; matched aftermarket element to the same rating. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
How often should the breather element be replaced?
Follow the manufacturer's interval and shorten it in dust. The useful comparison is with the air filter: a machine working in demolition or on a quarry floor loads both faster, and the breather is often the one that gets missed because it is not part of the routine air-side service. Where a machine has shown a rising particle count with no component wear, renew the breather element as part of the investigation.
Can the suction strainer be cleaned and reused?
Where the manufacturer permits cleaning a mesh strainer, yes, provided it is inspected afterwards for damage and for metal. On designs where the strainer is a pleated or bonded element, it is renewed. The important part of the job is not the cleaning itself but what the strainer collected, because that is the evidence about what is happening inside the system.
Why does the tank have positive pressure at all?
Because a breather is a balance, not a seal, and the reservoir volume changes with every cylinder stroke. Where a breather is restrictive or blocked, the tank holds pressure on the return stroke and vacuum on the extend stroke, which is why oil weeps from the cap and why the pump can be starved. A tank that hisses when the filler cap is released has told you the breather is not doing its job.