Bleeding Air from the Hydraulic System on the Hitachi EX120-3 Excavator

[MikePhua]

The EX120-3 and Its Hydraulic Legacy
The Hitachi EX120-3 is a mid-size hydraulic excavator introduced in the 1990s, part of the EX series that helped establish Hitachi’s reputation for precision, reliability, and smooth hydraulic control. With an operating weight of approximately 27,000 lbs and powered by the six-cylinder Isuzu BB-6BG1T diesel engine, the EX120-3 was widely adopted in construction, quarrying, and utility trenching across Asia, Europe, and North America.
Its hydraulic system is built around a variable displacement axial piston pump, pilot control circuits, and a load-sensing main valve block. The system is known for its responsiveness and durability, but like all hydraulic platforms, it is vulnerable to air intrusion during maintenance, hose replacement, or fluid changes.
Why Air in the Hydraulic System Is Dangerous
Air trapped in hydraulic lines or components can cause:

• Jerky or delayed actuator response
  
• Cavitation damage to pump and valve surfaces
  
• Inaccurate control feedback
  
• Reduced lifting or digging power
  
• Excessive noise and vibration
  
• Long-term seal degradation due to aerated fluid
  

In one case, a contractor in Queensland reported that his EX120-3 began to swing erratically after a hose replacement. The issue was traced to air trapped in the pilot circuit, which caused inconsistent spool movement in the swing valve.
When and Where Air Enters the System
Air can enter the hydraulic system through:

• Hose disconnection or rupture
  
• Low fluid levels in the reservoir
  
• Leaky suction lines or fittings
  
• Faulty return filters or cracked tank welds
  
• Improper fluid filling without venting
  

The most common entry point is during maintenance when the system is opened and not properly refilled or primed. Air can also be drawn in through worn pump seals or loose clamps on the suction side.
Bleeding Procedure for the EX120-3
To safely bleed air from the hydraulic system:

1. Check Fluid Level
   Ensure the hydraulic reservoir is filled to the correct mark with ISO 46 hydraulic oil or Hitachi-approved equivalent. Top off as needed.
   
2. Start Engine at Idle
   Run the engine at low RPM to allow gradual fluid circulation without pressure spikes.
   
3. Cycle All Functions Slowly
   Operate boom, arm, bucket, and swing functions gently to push air through the system. Avoid full-speed or high-load movements.
   
4. Hold Cylinders at Full Extension
   Extend each cylinder fully and hold for 10–15 seconds. Repeat with full retraction. This helps purge air from cylinder chambers.
   
5. Bleed Pilot Circuit Separately
   Locate pilot line bleeder screws near the control valve or pilot pump. Loosen slightly while operating controls to release trapped air.
   
6. Inspect Return Filter and Breather
   Ensure the return filter is clean and the tank breather is functional. A clogged breather can cause vacuum buildup and air ingestion.
   
7. Monitor for Foam or Milky Fluid
   If fluid appears aerated, allow machine to idle until bubbles dissipate. Do not operate under load until fluid clears.
   
8. Check for Leaks
   Inspect all fittings, hoses, and seals for signs of leakage. Tighten or replace as needed.
   

Additional Tips and Safety Measures

• Always bleed the system with the machine on level ground
  
• Use spill containment when loosening bleeder screws
  
• Wear eye protection and gloves during fluid handling
  
• Never operate the machine with known air in the system
  
• Replace damaged hoses with OEM-grade components
  

Some operators install transparent sight tubes on the return line to monitor fluid clarity during bleeding. Others retrofit pressure gauges on pilot lines to detect erratic pressure caused by air pockets.
Preventive Practices to Avoid Air Intrusion

• Maintain proper fluid levels daily
  
• Replace breather caps annually
  
• Use vacuum fill tools during major service
  
• Torque hose clamps and fittings to spec
  
• Avoid overfilling the reservoir, which can cause foaming
  

A fleet manager in Ontario implemented a checklist for hydraulic service that included air bleed verification. Over six months, system failures dropped by 40%, and machine responsiveness improved across the board.
Conclusion
Bleeding air from the hydraulic system of the Hitachi EX120-3 is a critical maintenance task that ensures smooth operation, protects components, and extends machine life. Whether after hose replacement or fluid change, a methodical approach to purging air—especially from pilot circuits and cylinders—can prevent costly downtime and restore full hydraulic performance. With proper technique and preventive care, the EX120-3 continues to deliver reliable power and precision in the field.