Excavator and Backhoe Loader (JCB) Hydraulic Tank: Breather, Cap, Strainer and Level Gauge

A hydraulic tank does four jobs on an excavator or backhoe loader: it holds the oil the pump draws, gives that oil somewhere to cool, lets trapped air rise out of it, and lets dirt settle below the level the pump picks up from. The parts that make those four jobs happen are small and easy to ignore, and there are only five worth knowing: the filler cap, the air breather, the suction strainer, the level gauge and the tank gasket.

Most operators meet the tank only once something has gone wrong. A cap that will not turn. Oil weeping down the side of the machine. A level that reads differently every time you look. Milky oil on the dipstick. Every one of those is a tank story rather than a pump story, and most are put right with a part you can hold in one hand.

This guide covers what the tank and its parts do, how to open a pressurised tank safely, how to read the level so the number means something, and what milky or foaming oil is telling you. It carries no prices, because the right cap or breather depends on the exact machine and a dealer quotes against that. If one of these parts is what brought you here, the construction equipment spare parts page is where to raise the request with your machine details.

What does the hydraulic tank on an excavator do?

The tank is the one place in a hydraulic system where the oil slows down. Everywhere else it is pushed hard through pumps, valves, hoses and cylinders. Inside the tank it gets a few seconds to shed heat, release the air mixed into it and drop the dirt it is carrying. How a tank is built therefore matters as much as how much it holds.

Danfoss puts numbers on the air part in its technical information for the Series T90 axial piston pumps. It recommends a reservoir sized to give the oil roughly 30 seconds of dwell time at maximum return flow, because that is how long entrained air needs to separate out. The return line should discharge below the normal oil level and point towards the middle of the tank. A baffle inside helps de-aeration and stops the oil surging as the machine slews. The outlet feeding the pump should sit above the floor, so gravity separation keeps large particles out of the suction line, with a 100 to 125 micron screen over that port.

How the parts sit inside a hydraulic tank

oil level

baffle

filler cap and air breather:
the only way air enters or leaves
return line,
below the oil level

to pump
suction strainer, raised off the floor

level gauge
(max and min)

drain plug
settled dirt stays here

• Filler cap and air breather on top: the only route for air in and out.
• Oil level: the line the gauge reads, which moves as the cylinders fill and empty.
• Return line discharging below the level and pointing inwards, so oil does not splash air in.
• Baffle: an internal wall that lengthens the path from return to pickup and stops the oil sloshing.
• Suction strainer on the pickup, held above the floor, clear of the settled dirt.
• Level gauge on the side wall, with maximum and minimum marks.
• Drain plug at the lowest point, where water and sludge collect.

Those rules explain a lot of everyday tank behaviour. Oil returning above the level beats air into the tank, which is why a return hose re-routed by a roadside mechanic can foam a tank that was fine for years. A pickup sitting too low pulls settled dirt back into the pump. Machines as different as the JCB 3DX backhoe loader and the Tata Hitachi EX 210 Infra excavator follow the same rules, with different tank shapes around them. How much oil each holds is a separate question, answered model by model in our guide to engine oil and hydraulic oil capacity.

Why is the hydraulic tank pressurised, and how do you release the pressure?

A sealed hydraulic tank builds pressure because oil expands as it heats. Over a shift the oil climbs well above the morning air temperature, takes up more room as it does so, and in a closed tank the air trapped above it has nowhere to go. By the time the machine stops, the tank holds a small positive pressure and the oil inside is hot enough to scald.

Construction machines are built both ways. Some tanks breathe through a filter element and sit at atmospheric pressure. Others are sealed, and Danfoss notes in the same document that a closed reservoir with no breather is adequate for most of the applications it covers. A sealed tank keeps dirt and humid air out completely, and the slight pressure helps push oil towards the pump inlet instead of making the pump suck for it.

The safe routine is the same on both kinds. Park on level ground, put the bucket flat down, stop the engine and let the oil cool. If the cap has an air-release button or bleed lever, press it and wait for the hiss to finish. If it does not, loosen the cap a quarter turn and wait until the hissing stops before taking it off. Hot oil under even a fraction of a bar leaves a half-open cap as a jet. The pressure your own machine runs at is printed in its operator manual, and it is not a number worth guessing.

What does the tank breather do, and what happens when it blocks?

The breather is the tank’s lung, and on a vented tank every litre of air moving in or out passes through it. When the boom, arm and bucket cylinders extend, oil leaves the tank and air has to come in to fill the space. When they retract, oil returns and the same volume of air has to get out. The breather lets that exchange happen without dust and water coming in with it.

Donaldson’s hydraulic reservoir accessories catalogue shows what a good element is expected to do. Its T.R.A.P. breather is rated to stop solid particles down to 3 micron at 97 per cent efficiency while keeping airborne moisture out, with water-holding material designed to regenerate each time oil returns. The catalogue also lists filler breathers that double as the fill point, with lockable caps, dipsticks and side-mounted versions.

A blocked breather shows itself in ways that look like bigger faults:

• The pump whines and the machine feels lazy, because it is pulling against a partial vacuum.
• The cap gasps inwards when you open it after a shift, or needs real force to turn.
• Oil is pushed out of the dipstick tube or the gauge seal, because a tank that cannot breathe out finds another way.
• Seals and hoses fail early, since each is now carrying pressure or vacuum it was never meant to see.

On Indian sites the dust reaches the breather first. A machine working a crusher yard in May pulls that air through a small element, and the monsoon adds humidity to the same job. A breather is cheap and takes minutes to change, so most dealers treat it as a service item alongside the other filters on the machine; on a backhoe loader it usually sits in the filter kit.

Suction strainer or return filter: what is the difference?

The suction strainer is a coarse mesh screen inside the tank that keeps large debris away from the pump, while the return filter is a fine replaceable element that cleans the oil coming back from the machine. Confusing them is how a pump gets starved: fit a fine element where a coarse screen belongs and the pump cannot draw through it.

Donaldson’s suction strainers use stainless steel mesh in sizes from 60 to 300 mesh, are cleanable and reusable, and are meant to sit at least 4 inches (102 mm) above the tank floor so they are not standing in the sludge. The catalogue says to clean one by swishing it in a non-caustic solvent and blowing it dry from the inside outwards, never the other way. Several models carry a bypass relief valve set at 3 psid (0.2 bar) or 5 psid (0.35 bar), so a clogged screen opens rather than starving the pump.

What pressure and flow all of this feeds is a separate subject, covered in our guide to excavator hydraulic pressure and pump flow. If you are shopping rather than servicing, compare the excavators and backhoe loaders listed in India and ask the dealer what the service schedule looks like before you sign.

How do you read the hydraulic oil level gauge correctly?

Read the level with the machine in the position its manual asks for, because the oil is shared between the tank and the cylinders. Extend the boom, arm and bucket and several litres sit out in the cylinders rather than the tank, so the gauge reads low and an operator adds oil the machine did not need. Bring everything back in and that oil returns, the tank is now overfull, and the extra is pushed out of the breather the first time it gets hot.

The routine that works on most machines is short. Park on level ground. Put the bucket flat down with the arm drawn in and the boom low, or whatever rest position the manual specifies. Stop the engine, wait for the foam to clear, then read the sight glass or draw the dipstick. Most excavators use a round or strip sight glass on the tank side with maximum and minimum marks; most backhoe loaders use a dipstick or a sight glass low on the tank. Read it at the same oil temperature each time, because warm oil sits higher than cold oil in the same tank.

Note the level in the same conditions every week, so a slow loss shows up as a trend instead of a surprise, and top up only with the grade the machine calls for. Our guide to hydraulic oil covers the grades.

Where does a hydraulic tank leak from?

A tank leaks at its joints far more often than through its steel. Work down the list below before deciding the tank itself has failed, because a weld repair is a workshop job and the other causes are not.

One warning. A wet tank side is not proof of a tank leak: oil from a hose, fitting or cylinder above it runs down the sheet metal and collects at the lowest ledge, which is usually the tank. Clean the area, run the machine, and trace the fresh oil to its highest point before ordering anything. Our guide to hydraulic hoses and fittings covers the joints above the tank.

What do milky oil and foam in the tank mean?

Milky, cloudy oil means water has got into the tank, and foam or small bubbles mean air has. They are different faults with different causes, and the tank is where both first become visible.

Water turns hydraulic oil from clear amber to the colour of weak tea with milk. It gets in through a tired breather drawing humid air, a pressure washer aimed at the filler cap or the gauge, condensation in a half-empty tank standing through a monsoon, or a cooler fault. Water rusts the tank from inside, strips the oil’s additives and ruins the thin film that keeps pumps and valves alive. It collects at the lowest point, which is why cracking the drain plug at a service to see what comes out first is worth two minutes.

Air makes the oil frothy and the machine spongy, with jerky cylinder movement and a pump that gets louder under load. The usual causes are a level that drops below the pickup somewhere in the work cycle, a loose or cracked fitting on the suction side, a return line discharging above the oil level, or a worn pump shaft seal. Both faults are reasons to stop rather than carry on. Running a machine on wet or aerated oil turns a cheap repair into a pump and valve repair, and the hydraulic system maintenance guide sets out the routine that prevents most of it. If the oil is also running hot, our guide to hydraulic oil temperature explains what to check next.

What should you check on the tank at every service?

Five minutes with the tank at every service catches most of what goes wrong with it, and none of these checks needs a tool more exotic than a rag and a torch.

• Level, in the machine’s specified rest position, on level ground, engine off.
• Oil condition in the sight glass or on the dipstick: clear amber is right, cloudy means water, frothy means air, dark and sharp-smelling means heat damage.
• Breather: whether it is caked in dust, and whether the cap opens and closes normally.
• Cap seal: supple rubber, no cracks, with the air release working on a pressurised tank.
• Gauge, plate and drain plug seals: dry metal around every one of them.
• Strainer, when the tank is next opened: cleaned, inspected, and the debris on it read as evidence rather than thrown away.

The debris is the part most people skip. Bright metal flakes mean something upstream is wearing. Black rubbery pieces mean a hose liner or a seal is breaking up. Sand means the tank has been open or the breather is finished. The strainer is the only place on the machine where you get to see what the oil has been carrying, and reading it once a year is cheaper than hearing it from the pump.

The excavator parts and components guide maps where the rest of it sits, and the parts and running costs guide collects the service items in one place. If you are weighing up a machine rather than maintaining one, compare the excavators and backhoe loaders listed in India and connect with a dealer near you.

Sources: Danfoss, Technical Information: Series T90 Axial Piston Pumps, Size 055/075/100; Donaldson, Hydraulic Reservoir Accessories. Service figures for your own machine must be taken from its operator manual. Specifications and features are indicative, vary by variant, location and date, and should always be confirmed with the official OEM or authorised dealer before any purchase decision. DesiMachines is not liable for decisions taken on the basis of information that may have changed after publication.