Bleeding the Hydraulic Pump on Hitachi Mini Excavators

[MikePhua]

The Hitachi Mini Excavator Line and Its Hydraulic System
Hitachi Construction Machinery, founded in 1970, has long been a leader in hydraulic excavator design. Their mini excavator series—ranging from the ZX17U to the ZX60U—has earned a reputation for precision, reliability, and compact power. These machines are widely used in urban construction, landscaping, and utility trenching, with thousands sold across Asia, Europe, and North America.
At the heart of every Hitachi mini excavator is a closed-center hydraulic system powered by a variable displacement axial piston pump. This pump supplies pressurized fluid to the boom, arm, bucket, travel motors, and auxiliary circuits. When air enters the system—typically after filter changes, hose replacements, or fluid loss—the pump must be properly bled to restore full function and prevent cavitation damage.
Terminology Notes

• Hydraulic Cavitation: The formation of vapor bubbles in hydraulic fluid due to low pressure, which can damage pump components.
  
• Charge Pressure: The baseline pressure supplied to the pump inlet to prevent cavitation.
  
• Bleed Port: A designated fitting or plug used to release trapped air from the hydraulic circuit.
  
• Pilot Circuit: A low-pressure control system that actuates valves and directional flow in the main hydraulic system.
  

Symptoms of Air Entrapment and Pump Starvation
After hydraulic service or fluid loss, operators may notice:

• Slow or jerky boom and arm movement
  
• Whining or growling noise from the pump
  
• Inability to lift or travel under load
  
• Delayed response to joystick input
  
• Visible foaming in the hydraulic reservoir
  

In one case, a contractor in British Columbia replaced a hose on his ZX30U and refilled the tank, but the machine refused to lift the boom. After checking for leaks and verifying fluid level, he bled the pump manually and restored full function within minutes.
Bleeding Procedure and Safety Precautions
Bleeding the hydraulic pump on a Hitachi mini excavator requires careful attention to pressure and cleanliness. The general procedure includes:

• Park the machine on level ground and lower all implements
  
• Shut off the engine and release residual pressure via the pilot circuit
  
• Locate the bleed port on the pump housing or inlet line
  
• Loosen the port slowly and allow air to escape until fluid flows steadily
  
• Tighten the port and restart the engine
  
• Cycle all hydraulic functions gently to purge remaining air
  

Precautions:

• Wear eye protection and gloves—hydraulic fluid under pressure can cause injury
  
• Use clean tools and avoid introducing debris into the system
  
• Never bleed the pump while the engine is running unless specified by the manufacturer
  
• Monitor fluid level during and after bleeding—air pockets can cause false readings
  

Some newer Hitachi models include an automatic bleed function via the monitor panel, but manual bleeding remains essential after major service.
Additional Checks and Troubleshooting
If bleeding does not resolve the issue, consider:

• Inspecting suction lines for cracks or loose clamps
  
• Verifying that the return filter is not clogged
  
• Checking pilot pressure with a gauge (typically 400–600 psi)
  
• Testing main pump discharge pressure under load
  
• Inspecting the charge pump for wear or contamination
  

A technician in Ohio diagnosed a persistent lift issue on a ZX40U and found that the suction strainer inside the tank was partially blocked by seal fragments. Cleaning the strainer and bleeding the pump restored full hydraulic response.
Preventive Maintenance and Long-Term Reliability
To reduce the need for bleeding and protect the hydraulic pump:

• Replace hydraulic filters every 500 hours or as specified
  
• Use OEM-grade fluid with correct viscosity and anti-foaming properties
  
• Inspect hoses and clamps quarterly
  
• Avoid running the machine with low fluid levels
  
• Train operators to recognize early signs of cavitation or air intrusion
  

Fleet managers often install sight gauges and pressure sensors to monitor system health. One rental company in Texas reduced hydraulic failures by 35% after implementing a post-service bleed checklist.
Recommendations for Technicians and Operators

• Keep a dedicated bleed kit with wrenches, rags, and sealant
  
• Document service intervals and fluid changes
  
• Use torque specs when reinstalling bleed plugs or fittings
  
• Avoid overfilling the reservoir—excess fluid can aerate under vibration
  
• Educate operators on proper shutdown and restart procedures after service
  

Conclusion
Bleeding the hydraulic pump on a Hitachi mini excavator is a critical step in restoring system integrity after fluid loss or component replacement. With a structured approach and attention to detail, technicians can prevent cavitation, improve response time, and extend pump life. These compact machines rely on precise hydraulic control—and even a small air pocket can disrupt performance. Proper bleeding ensures that the system runs smooth, quiet, and strong, ready for the next trench, lift, or grade.