How to Get Air Out of Hydraulic System Before It Costs You a Cylinder Rebuild
If your loader arm bounces on the way up, your bucket curls in jerks instead of one smooth motion, or the controls feel mushy in your hand—you're probably not looking at a mechanical failure. You're looking at compressible gas where it shouldn't be. Air compresses readily, but hydraulic fluid doesn't—and once those bubbles get into the lines, every command you give the machine has to fight them first. The good news: you can usually fix this in your driveway with a wrench and twenty minutes. The bad news: if air keeps coming back after you bleed it, something else is wrong—usually a tired pump shaft seal on the suction side, or a cylinder pulling air past its rod seal. Knowing how to get air out of hydraulic system is the first skill. Reading what the system tells you afterward is the second.
Table of Contents:
- How Air Enters a Hydraulic System
- Telltale Signs of Trapped Air in Hydraulic Fluid
- What Trapped Air Does to System Components
- How to Bleed Air from the Hydraulic Circuit
- When Air Bubbles Keep Coming Back
- Preventing Air Ingress and Protecting System Performance
- When Bleeding Isn't Enough
- FAQ
How Air Enters a Hydraulic System
Air enters during maintenance, fluid changes, or through leaks in seals, hoses, or fittings. Whenever you crack a line open, swap a filter, or top off the reservoir, you're inviting air into a circuit designed to keep it out. Bleeding is part of the job.
Other ways are less obvious. The suction side of the pump runs below atmospheric pressure—so a crack in the suction hose, loose fitting, or worn O-ring on the inlet line lets air in, even with no oil leaking out. Low fluid levels expose the suction line, drawing air in on every revolution. Intake restrictions—clogged strainer, partially closed valve, collapsed hose—create excess vacuum that pulls air through fittings tight enough to seal oil but not air. A worn pump shaft seal on the suction side aerates the fluid every time the pump runs.
Tip from the Skidsteers.com team: After every fluid change or filter swap, run the machine at low idle for 30 seconds before exercising the implements. That short window lets the fluid settle and lets any air introduced during service rise to the top of the reservoir before it gets pumped through the circuit.
Telltale Signs of Trapped Air in Hydraulic Fluid
Before chasing a problem, make sure it's actually air. The common symptoms that indicate you need to get air out of hydraulic system components include:
| Symptom | Operational Impact |
|---|---|
| Spongy Response | Delayed cylinder response; controls feel soft instead of crisp. |
| Erratic Movement | Jerky implement movement, especially during initial travel. |
| Pump Noise | Rattling or marble-like sounds; sharp knocking (dieseling). |
| Reservoir Foam | Persistent bubbles in the sight glass that don't clear. |
| High Temperature | Fluid temperature climbing faster than usual under normal load. |
What Trapped Air Does to System Components
It's tempting to treat air as a minor nuisance. It's not. Aeration (air entering from outside) and cavitation (vapor bubbles forming due to pressure drops) are different mechanisms with different damage profiles. Cavitation does the most violent mechanical work: as vapor cavities move into higher-pressure regions, they implode with enough force to pit the inside of pumps and valves. That metal then circulates as contamination.
Aeration's damage is more about chemistry and lubrication. Air-rich fluid lubricates worse, runs hotter, and degrades faster—fluid breakdown accelerates, seals burn, and the system becomes less efficient. Both shorten component life and both are fixable if you catch them early.
How to Bleed Air from the Hydraulic Circuit
Effectively removing air means purging trapped bubbles from the highest points of the circuit by cycling actuators while the system is under low pressure. Air rises, fluid follows, gravity does the work.
Start with the machine on level ground, engine off, implements lowered. Before bleeding, fill the reservoir to the manufacturer's recommended level—bleeding while the tank is low is the quickest way to reintroduce air the moment you start the engine.
Most skid steer, loader, and compact equipment cylinders are designed to self-bleed when cycled through their full range. Run each function (lift, tilt, auxiliary) end to end several times at low engine RPM and watch for symptoms to fade.
If your cylinder does have a bleeder screw, use it for stubborn air pockets. As a last resort, you can crack the highest line fitting while the system is at very low pressure. With the engine at low idle, slowly loosen the bleed point while operating the controls until a steady stream of bubble-free fluid appears. Don't try to force air out with high pressure—it pushes air into solution where it won't bleed back out, and in extreme cases can cause dieseling, when air compressed in a cylinder ignites the oil vapor and damages seals.
Tip from the Skidsteers.com team: Keep a length of clear vinyl tubing (1/4-inch or 5/16-inch ID works for most fittings) and a pair of hose clamps in your toolbox. They turn the bleed-screw-or-fitting step into a five-minute job—you can see exactly when the bubbles stop, instead of guessing.
When Air Bubbles Keep Coming Back
This is where most operators get frustrated. You bleed the system, the bucket works for a day, and by week's end the loader arm is bouncing again. That's air re-entering somewhere.
The first suspect is reservoir fluid level. If it's dropping, you have an external leak. Worth understanding: pressure-side leaks let oil out but don't let air in. Suction-side leaks do the opposite—they let air in without losing oil, because the suction line runs below atmospheric pressure.
The second most common cause on high-hour machines is a worn pump shaft seal on the suction side. The third is a cylinder retracting under load and developing negative pressure on the rod side—usually a faulty load-control valve, made worse by worn rod seals. If a system has been bled properly and symptoms return within hours, stop bleeding and start diagnosing the source of ingress.
Preventing Air Ingress and Protecting System Performance
Regular inspections dramatically reduce the likelihood of air contamination. Check fluid levels at the start of every shift. Use the manufacturer's recommended hydraulic fluid type—mixing oils can neutralize anti-foam additives, leaving the fluid prone to foaming. Watch the reservoir for foam after running the machine hard. Vigilant seal maintenance is the step most operators skip. A pump shaft seal that's seeping doesn't fail dramatically—it slowly aerates the system until symptoms become impossible to ignore.
When Bleeding Isn't Enough
Cylinder rod seals, pump shaft seals, and the O-rings buried in fittings all have a service life. Once they're past it, no amount of bleeding keeps the system tight for long. Skidsteers.com stocks hydraulic cylinders and replacement seal kits for the most common skid steer, loader, and compact equipment platforms—including the rebuild kits that turn a chronically aerated machine back into one that holds pressure between bleeds.
FAQ
How long does it take to bleed air from a hydraulic system?
For a typical skid steer where cycling alone clears the air, plan on 15–30 minutes. If you have to crack fittings or use bleeder screws, add another 10–20 minutes per circuit. If the machine still feels spongy after an hour, you're likely chasing a leak that keeps replacing the air.
What's the difference between entrained air and dissolved air?
Dissolved air is held within the fluid at the molecular level and doesn't cause problems. Entrained air is visible as foam or bubbles and causes spongy controls and noise. The two convert back and forth depending on pressure changes.
Can trapped air cause permanent damage to hydraulic components?
Yes. Reduced lubrication from air-rich fluid wears surfaces faster, and cavitation pits pump internals. Catching it early with a 20-minute bleed prevents a costly pump replacement later.
Why does my hydraulic system overheat after I notice air symptoms?
Aerated fluid carries heat poorly. Also, the pump does extra work compressing air bubbles without moving more fluid—that wasted work converts directly into heat, often outpacing the cooling system.
What's a restricted intake line, and how do I check for it?
It's the suction line from reservoir to pump. If blocked by a clogged strainer or collapsed hose, it creates a vacuum that pulls air past seals. Check for obstructions with the engine off and ensure the hose feels firm, not soft or kinked.