If you're currently struggling with bleeding air from closed loop hydraulic system components, you already know how frustrating that "spongy" feeling can be when you move a joystick or hit a pedal. It's one of those maintenance tasks that sounds simple on paper but can turn into a real headache if the air just refuses to budge. Unlike open systems where air eventually finds its way back to a large reservoir and bubbles out, a closed loop (like a hydrostatic drive) is much more stubborn. If that air stays trapped, you're looking at more than just sluggish performance; you're looking at potential pump damage from cavitation.
Why trapped air is such a nightmare
Air is basically the enemy of any hydraulic setup, but it's especially nasty in a closed loop. The whole point of hydraulics is that liquid doesn't compress, which is how we get that instant, powerful force. Air, on the other hand, is very squishy. When you have air bubbles in the lines, the energy from the pump goes into compressing those bubbles instead of moving the piston or motor.
Beyond the loss of power, there's the noise. If your machine sounds like it's chewing on a bag of marbles, that's usually air being hammered by high pressure. This can lead to "dieseling," where the air bubbles get compressed so fast they actually explode, pitting the metal surfaces inside your expensive pumps and motors. It's not just annoying; it's expensive.
Before you start cracking lines
The first thing you'll want to do is make sure you aren't fighting a losing battle. There is no point in bleeding air from closed loop hydraulic system parts if the system is just sucking more air back in. Check your intake hoses and those pesky O-rings. Even a tiny pinhole leak on the suction side won't necessarily leak oil out, but it will absolutely suck air in like a straw with a crack in it.
Also, check your fluid level. It sounds obvious, I know, but you'd be surprised how many people try to bleed a system when the reservoir is just low enough that the pump is creating a vortex and pulling in fresh air every time the machine tilts. Fill it up to the "full" mark—or even slightly over if the manufacturer allows it for the bleeding process—to give those bubbles plenty of room to escape.
The basic process for closed loops
In a closed loop system, the oil mostly just cycles between the pump and the motor. To get the air out, we usually rely on the charge pump. This is the smaller pump that keeps the main loop pressurized and replaces the oil that leaks out for cooling and lubrication.
1. The initial "burp"
Start by filling the pump and motor cases with clean oil before you even turn the engine over. If you've just replaced a component, never start it dry. Once everything is hooked up, disable the engine so it can't actually fire—unplug the solenoid or the ignition—and crank it over in short bursts. This lets the charge pump start moving oil through the system without the massive pressure that comes when the engine is running at full tilt.
2. Cycling the controls
Once you've got some oil moving, start the engine and let it idle. Don't go straight to full throttle. Slowly move your controls back and forth. You're not looking for big movements yet; just nudge the motor or cylinder in one direction, then the other. You'll probably hear some groaning or whining—that's normal. What you're doing here is encouraging the trapped air to move toward the case drains or the flushing valves where it can eventually reach the reservoir.
3. Cracking the fittings (The messy part)
If the air is being particularly stubborn, you might need to "crack" a fitting. This is where you slightly loosen a high-point connection while the system is under very low pressure. Be extremely careful here. Hydraulic fluid under pressure can be dangerous. You only want to loosen it enough for a little bit of foamy oil to hiss out. Once the foam turns into a solid stream of clear oil, tighten it back up immediately.
Why closed loops are different
In an open loop system, the oil goes back to the tank after every cycle, which makes bleeding relatively easy. In a closed loop, the oil just keeps spinning in a circle. This is why the charge pressure is so critical. If your charge pressure is too low, it won't be strong enough to "push" the air bubbles out of the main loop and into the cooling circuit.
If you've been cycling the controls for twenty minutes and it still feels like mush, you might need to check your charge pressure relief valve. If that valve is stuck open or set too low, you'll never get the air out because there isn't enough "oomph" to move it.
Common places air hides
Air loves to get stuck in high spots. If you have a hose that loops upward and then back down, that's a prime spot for an air pocket. Sometimes, you might actually have to unbolt a component or a hose and move it lower than the rest of the system just to get the bubble to move along.
Another culprit is the motor case. Most closed loop motors have a case drain line that goes back to the tank. If this line is restricted or if the motor is mounted in a way that creates an air pocket at the top of the housing, you'll have a hard time getting it quiet. Some motors have multiple drain ports—make sure the one at the very top is the one being used for the return line.
Using the "Bleed Valves"
Some modern systems are fancy enough to include dedicated bleed valves. If you're lucky enough to have these, use them! Usually, they're located near the top of the pump or on the manifold. You just crack them open, run the charge pump, and wait for the bubbles to stop. It's a lot cleaner than loosening a JIC fitting and getting oil all over your boots.
Tips for a successful bleed
- Patience is a virtue: Don't rush it. Rushing usually leads to over-revving the engine, which just whips the air into a fine froth. Frothy oil is much harder to bleed than big bubbles. If the oil looks like a milkshake, shut the machine down and go have a coffee for an hour to let the bubbles settle.
- Heat helps: Cold oil is thick and holds onto air. Once you've got the major air pockets out, let the system warm up a bit. Thinner, warm oil releases air much more easily.
- Watch the reservoir: While you're bleeding air from closed loop hydraulic system lines, keep an eye on that tank. As air leaves, the oil level will drop. If it drops too far, you'll just suck more air in, and you'll be right back where you started.
Safety first, always
I can't stress this enough: hydraulic injection injuries are life-changing. Never use your hand to feel for a leak or to see if air is hissing out of a fitting. Use a piece of cardboard or wood. If you're cracking a line, wear safety glasses and heavy gloves. Even a low-pressure charge circuit can have enough pressure to cause problems if a fitting pops off.
Wrapping it up
Getting all the air out of a closed loop isn't always a "one and done" job. Sometimes you'll get the machine running 90% right, only to find a tiny bit of sponginess remains. Often, after an hour or two of actual work, that last little bit of air will finally find its way to the reservoir.
Just remember: keep the oil clean, keep the reservoir full, and don't be afraid to let the machine sit and rest if the oil gets too foamy. It takes a little bit of a "feel" for the machine, but once you hear that pump go from a high-pitched scream to a low, smooth hum, you'll know you've finally nailed the process of bleeding air from closed loop hydraulic system setups. It's a satisfying feeling, and your hardware will definitely thank you for it in the long run.