In a perfect hydraulic system, the fluid is incompressible, allowing for the instantaneous and precise transmission of force. However, when air enters the system, it acts like a “spring,” compressing under pressure and causing a variety of operational headaches.
Bleeding air is a critical maintenance task that ensures the safety, efficiency, and longevity of your equipment.
Why Must You Bleed Air? (The “Why”)
Air is the enemy of hydraulic efficiency. If left inside the circuit, it leads to several damaging phenomena:
- Spongy or Erratic Operation: Because air is compressible, the cylinder may “jerk” or hesitate instead of moving smoothly. This is especially dangerous in precision lifting or steering applications.
- Cavitation and Component Damage: As air bubbles travel from low-pressure areas to high-pressure zones (like inside a pump), they implode violently. These micro-explosions can pit metal surfaces, leading to premature pump failure.
- Overheating: Compressing air generates significant heat. This localized “hot spot” can degrade the hydraulic oil and damage sensitive seals.
- Noise (Aeration): A system with air often produces a high-pitched whining or “growling” sound, indicating that the pump is struggling and the fluid is foaming.
How to Bleed Air from the System (The “How”)
There are several methods to remove trapped air, depending on the complexity of the machine and the components involved.
1. The Cycling Method (Self-Bleeding)
Most modern hydraulic systems are designed to be “self-bleeding” if the air is trapped in the lines or cylinders.
- The Process: Run the engine or motor at a low idle. Gently extend and retract the hydraulic cylinders through their full stroke multiple times (at least 5–10 cycles).
- The Goal: This pushes the air back through the return lines into the reservoir, where the bubbles can rise to the surface and escape through the breather.
- Note: Do not “deadhead” the cylinder (hold it at full pressure) for too long, as this can heat the aerated oil further.
2. Using Bleed Screws (Manual Bleeding)
Some high-performance cylinders or hydraulic motors come equipped with dedicated bleed valves or screws at the highest point of the component.
- The Process: With the system under very low pressure, slightly loosen the bleed screw. Watch for a “milky” or foamy mixture of oil and air to escape.
- The Result: Once the fluid runs clear and steady (no more bubbles), tighten the screw immediately.
3. Bleeding the Pump (Priming)
If a pump is “air-locked,” it cannot draw oil from the tank.
- The Process: Loosen the pressure-side fitting slightly while the pump is off. Allow gravity to push oil through the suction line until oil starts to drip from the fitting. Tighten the fitting and then start the motor briefly to check for flow.
4. Checking the Suction Line
If air keeps returning after you bleed it, you likely have a “suction leak.”
- The Cause: A loose fitting or a cracked hose on the intake side of the pump. Even if oil isn’t leaking out, the vacuum of the pump can suck air in.
Summary Checklist for Air Removal
- Check Oil Level: Always ensure the reservoir is full before bleeding; a low oil level is the #1 cause of air entering the system.
- Monitor the Reservoir: Look for foam. If the oil looks like a “latte,” let the machine sit for 30 minutes to allow the bubbles to dissipate naturally.
- Inspect the Breather: Ensure the tank breather is clean so air can actually escape the reservoir.
