Understanding Hydraulic Fluid Leaks in John Deere 720 Tractors

[MikePhua]

Introduction
The John Deere 720, a two-cylinder tractor introduced in the late 1950s, has become a staple in vintage tractor collections and agricultural operations. However, as with any aging machinery, maintenance challenges arise. One common issue faced by owners is the leakage of hydraulic fluid into the transmission, leading to overfilled transmission cases and potential operational problems.
The Hydraulic System and Transmission in the 720
The 720's hydraulic system is designed to operate independently of the transmission. The hydraulic fluid is stored in a separate reservoir and is used to power implements and other attachments. The transmission, on the other hand, relies on gear oil for lubrication and operation.
Despite these separate systems, there are points where the hydraulic system interfaces with the transmission. One such point is the area between the differential housing and the rockshaft housing. This cavity is intended to house 90W gear oil at atmospheric pressure. However, components like the PTO drive housing and the load and depth control assembly protrude into this cavity, creating potential pathways for hydraulic fluid to enter.
Common Causes of Hydraulic Fluid Leaks into the Transmission

1. Worn or Damaged Seals: Over time, seals can degrade, allowing hydraulic fluid to seep into unintended areas. The load and depth control assembly's snout is a known weak point where seals may fail, leading to hydraulic fluid entering the differential cavity.
   
2. Gasket Failures: The gasket between the axle housing and transmission housing can deteriorate, especially if exposed to contaminants or excessive pressure. This deterioration can result in leaks, allowing hydraulic fluid to mix with gear oil.
   
3. Overfilled Hydraulic System: If the hydraulic system is overfilled, excess pressure can force hydraulic fluid into the transmission through compromised seals or gaskets.
   

Symptoms of a Leak

• Overfilled Transmission: A noticeable increase in the transmission fluid level, often accompanied by fluid leaking from the vent behind the cab.
  
• Contaminated Fluid: The presence of hydraulic fluid in the transmission, which may appear foamy or discolored.
  
• Operational Issues: Slipping gears or erratic shifting can occur due to the contamination of gear oil with hydraulic fluid.
  

Diagnosing the Issue
To accurately diagnose the source of the leak:

1. Inspect Seals and Gaskets: Examine the load and depth control assembly snout and the gasket between the axle and transmission housing for signs of wear or damage.
   
2. Check Fluid Levels: Ensure that both the hydraulic and transmission fluid levels are within the recommended ranges. An overfilled hydraulic system can exacerbate the issue.
   
3. Monitor Fluid Condition: Assess the condition of the transmission fluid. If it appears milky or has a foamy texture, it's likely contaminated with hydraulic fluid.
   

Repair and Maintenance Recommendations

1. Replace Faulty Seals and Gaskets: If worn or damaged seals and gaskets are identified, replace them promptly to prevent further leakage.
   
2. Flush the Transmission: After addressing the source of the leak, flush the transmission to remove any contaminated fluid. Refill with the appropriate gear oil to restore proper lubrication.
   
3. Regular Maintenance: Implement a routine maintenance schedule to inspect seals, gaskets, and fluid levels. Regular checks can help identify potential issues before they lead to significant problems.
   

Conclusion
Hydraulic fluid leaks into the transmission of John Deere 720 tractors are a manageable issue with proper diagnosis and maintenance. By understanding the system's design and common failure points, owners can take proactive steps to ensure the longevity and optimal performance of their equipment. Regular inspections and timely repairs are key to keeping these vintage tractors running smoothly for years to come.