Resolving Hydraulic Arm Lag on the Kubota U35 While Operating a Brush Cutter

[MikePhua]

Understanding the Problem: Arm Extension Delay Under Load
The Kubota U35 Super Series excavator is a compact powerhouse, often paired with hydraulic attachments like brush cutters for vegetation management. However, some operators have reported a peculiar issue: when the brush cutter is engaged, all machine functions operate normally except for one—the arm extends outward extremely slowly. Retraction remains unaffected, and when the cutter is turned off, the arm resumes normal movement.
This behavior suggests a hydraulic flow prioritization conflict, where the brush cutter’s demand for fluid disrupts the balance of flow to the arm’s extension circuit.
Hydraulic System Architecture and Flow Dynamics
The U35’s auxiliary hydraulic system is designed to support high-flow attachments. It includes:

• A variable displacement hydraulic pump
  
• Flow control valves for auxiliary circuits
  
• A return-to-tank port for low-pressure fluid
  
• Priority valves that allocate flow based on demand
  

When the brush cutter is running, it draws maximum flow from the auxiliary circuit. If the system lacks a dedicated priority valve for boom and arm functions, the cutter may monopolize available flow, leaving insufficient pressure for arm extension—especially since extending the arm requires more force than retracting it due to leverage and gravity.
Key Terminology
- Auxiliary Hydraulic Flow: The portion of hydraulic output dedicated to attachments like brush cutters, hammers, or augers.
- Priority Valve: A hydraulic component that ensures critical functions (e.g., boom lift, travel) receive fluid before auxiliary circuits.
- Return-to-Tank Port: A low-pressure return path that allows fluid from attachments to bypass the main valve block and flow directly to the reservoir.
- Flow Limiter: A setting or valve that restricts the maximum flow to auxiliary circuits to prevent system overload or imbalance.
Root Cause Analysis
The issue likely stems from one or more of the following:

• The brush cutter is consuming nearly all available hydraulic flow, starving the arm extension circuit.
  
• The return-to-tank configuration allows fluid to bypass the valve block, reducing backpressure and affecting flow balance.
  
• The system lacks a priority valve for arm extension, or the valve is malfunctioning.
  
• The flow limiter is set too high, allowing excessive flow to the cutter.
  

Field Case: Minnesota Brush Clearing
An operator in Minnesota encountered this exact issue while clearing brush with a U35. With the cutter engaged, the arm would retract normally but extend sluggishly. After consulting with a dealer, he was advised to reduce the maximum flow setting to the auxiliary circuit. Once adjusted, the arm regained normal speed—even with the cutter running. This confirmed that the cutter’s flow demand was overwhelming the system.
Recommended Solutions and Adjustments
To resolve or prevent this issue, consider the following steps:
- Reduce auxiliary flow using the machine’s onboard settings or manual flow limiter valve.
- Inspect the hydraulic schematic to verify the presence and function of priority valves.
- Ensure the return-to-tank port is configured correctly and not bypassing critical circuits.
- Install a flow divider or priority valve if one is absent or malfunctioning.
- Use attachments with built-in flow control or pressure relief to prevent system overload.
- Test the system under load with a pressure gauge to identify pressure drops or flow starvation.
Suggested Parameters for Optimal Operation

• Auxiliary flow setting: 50–70% of maximum when using high-demand attachments
  
• System pressure: Maintain above 2,500 psi during simultaneous arm and cutter operation
  
• Return line backpressure: Should not exceed 100 psi to avoid flow disruption
  
• Arm cylinder extension rate: Should match retraction rate within 10–15% under load
  

Preventive Maintenance and Best Practices
To avoid hydraulic conflicts and ensure smooth operation:

• Regularly inspect and clean hydraulic filters and screens
  
• Monitor fluid temperature during extended cutter use; overheating can reduce efficiency
  
• Use OEM-approved hydraulic fluid with proper viscosity for ambient conditions
  
• Train operators to adjust flow settings based on attachment type and task
  
• Document attachment performance and hydraulic behavior for future reference
  

Anecdote: The Cutter That Stole the Flow
In Ontario, a contractor outfitted his U35 with a high-flow brush cutter for clearing roadside vegetation. Initially, the machine performed well, but after a few hours, the arm began lagging. Suspecting a mechanical fault, he replaced the arm cylinder—only to find the issue persisted. A hydraulic technician later discovered the cutter’s flow demand exceeded the pump’s capacity, and the return-to-tank setup was bypassing the arm circuit. After installing a flow divider and reducing the cutter’s flow setting, the machine returned to full functionality.
Conclusion: Balancing Flow for Seamless Operation
The Kubota U35 is a capable excavator, but like all hydraulic systems, it requires careful flow management when paired with demanding attachments. Arm extension lag during brush cutter use is not a failure—it’s a symptom of flow prioritization. By adjusting auxiliary settings, inspecting valve configurations, and understanding the system’s architecture, operators can restore full performance and keep their machines working efficiently in the field.