Diagnosing Hydraulic Drive Issues on the ASV MD-70 Positrack Loader

[MikePhua]

Introduction to the ASV MD-70
The ASV MD-70 is a compact track loader designed for grading, excavation, and light earthmoving tasks. Known for its rubber track undercarriage and maneuverability, the MD-70 is favored by contractors working in soft terrain or confined spaces. However, like many hydraulic-driven machines, it can suffer from performance degradation over time—particularly in its track drive system.
One common issue reported by operators is poor track power, especially after driveline repairs or extended use. Understanding the hydraulic architecture and recognizing symptoms of internal bypassing are key to restoring full functionality.
Hydraulic Drive System Overview
The MD-70 uses a hydrostatic drive system composed of:

• Hydraulic drive motors (left and right)
  
• A main hydraulic pump
  
• Case drain lines returning to the reservoir
  
• Control valves for forward and reverse actuation
  

Each drive motor receives pressurized fluid to rotate the tracks. Excess fluid and internal leakage are routed through the case drain line—a small third hose that returns oil to the reservoir.
Symptoms of Internal Bypass and Drive Weakness
When the machine exhibits poor track power, especially after replacing the driveline, the following symptoms may appear:

• Oil pouring from the case drain line during forward or reverse engagement
  
• Weak or sluggish movement despite throttle input
  
• Uneven track response between left and right sides
  
• No visible external leaks or hose damage
  

These signs suggest internal bypassing within the hydraulic drive motors. In other words, pressurized fluid is leaking past internal seals and returning prematurely to the tank, reducing torque output.
Understanding the Case Drain Line
The case drain line is designed to relieve internal pressure and return low-pressure fluid to the reservoir. However, excessive flow through this line indicates that the motor is no longer sealing properly. This can be caused by:

• Worn internal seals or bearings
  
• Scored motor housing or rotor
  
• Contaminated hydraulic fluid
  
• Overheating or cavitation damage
  

In one documented case, a contractor noticed a steady stream of oil from the case drain line after engaging the drive circuit. The machine could barely climb a small grade. After removing the drive motor and inspecting the internals, he found a scored rotor and degraded seals—classic signs of internal bypass.
Are the Drive Motors Rebuildable?
Yes, most hydraulic drive motors used in compact loaders are rebuildable, provided the housing is intact and parts are available. Rebuilding typically involves:

• Replacing seals, bearings, and wear rings
  
• Cleaning or resurfacing the rotor and stator
  
• Pressure testing the motor before reinstallation
  

However, rebuilding requires precision tools and hydraulic knowledge. If the motor housing is cracked or deeply scored, replacement may be more cost-effective.
Removal and Replacement Tips
Removing the drive motors from the MD-70 involves:

• Disconnecting hydraulic lines (mark them to avoid confusion)
  
• Unbolting the motor from the frame or final drive assembly
  
• Draining residual fluid and inspecting for contamination
  
• Checking couplers and splines for wear
  

Before installation, flush the hydraulic system to remove debris and ensure clean fluid reaches the new or rebuilt motor.
Recommendations for Long-Term Reliability
To prevent future drive issues and extend motor life:

• Replace hydraulic fluid and filters every 500 hours or annually
  
• Use OEM-spec fluid with proper viscosity and anti-foaming additives
  
• Inspect case drain flow periodically—excessive flow is an early warning
  
• Avoid prolonged idling with drive circuits engaged
  
• Keep track tension within manufacturer specs to reduce motor strain
  

Anecdote: The Vineyard Loader That Lost Its Pull
In central California, a vineyard operator used an MD-70 to grade rows and haul compost. After replacing the driveline, he noticed the machine struggled to move uphill. Oil poured from the case drain line, and the tracks barely responded. A local technician diagnosed internal bypass in the left drive motor. After rebuilding the motor and flushing the system, the loader regained full power—just in time for harvest season.
Finding Technical Support and Manuals
While service manuals for older ASV models can be hard to locate, many technicians recommend:

• Contacting ASV factory support directly
  
• Searching aftermarket parts sites for downloadable PDFs
  
• Joining regional equipment forums or user groups for shared resources
  

Having a manual on hand simplifies troubleshooting and ensures correct torque specs, fluid types, and disassembly procedures.
Conclusion: Restoring Power Through Precision Diagnosis
The ASV MD-70 is a capable machine, but like any hydraulic loader, its performance depends on the integrity of its drive motors and fluid system. When track power fades and oil pours from the case drain, it’s time to look inside—not just at hoses and fittings, but at the motor’s internal health. With careful inspection, proper rebuilds, and preventive maintenance, the MD-70 can return to full strength and keep pushing through the toughest terrain.