Final Drive Seal Failure and Hydraulic Leak Repair on the Kubota U35S2

[MikePhua]

The U35S2 and Its Compact Excavator Lineage
The Kubota U35S2 is part of Kubota’s U-series compact excavator family, designed for tight-space operation with zero tail swing and efficient hydraulic performance. With an operating weight around 3.5 metric tons and a dig depth exceeding 10 feet, the U35S2 balances maneuverability and power for urban trenching, landscaping, and utility work. Kubota’s compact excavators have gained global popularity since the 1990s, with tens of thousands sold across Asia, Europe, and North America. The U35S2, though less documented than newer dash-4 models, remains a reliable workhorse in small contractor fleets.
Its final drive system—responsible for track propulsion—is a sealed hydraulic motor assembly mounted within the track frame. These drives are vulnerable to seal wear, especially under abrasive conditions or after long-term storage. A hydraulic leak from the final drive often signals internal seal failure, which can lead to contamination, pressure loss, and eventual drive damage.
Identifying Hydraulic Leaks and Seal Access Points
Operators may notice:

• Hydraulic fluid dripping from the rear or bottom of the final drive
  
• Wetness around the motor housing or sprocket hub
  
• Reduced travel speed or uneven track response
  
• Fluid level drop in the main hydraulic reservoir
  
• No visible hose damage, suggesting internal leakage
  

The U35S2’s final drive includes a multi-piece housing with a removable rear cover secured by Allen bolts. Behind this cover lies the primary seal stack, which separates the gear oil chamber from the hydraulic motor. When this seal fails, hydraulic fluid can migrate into the gear case or leak externally.
Disassembly Strategy and Seal Replacement
To access and replace the seal:

• Safely lift the machine and remove the track from the affected side
  
• Clean the final drive exterior to prevent contamination
  
• Remove the Allen bolts securing the rear cover
  
• Gently pry off the cover, avoiding damage to mating surfaces
  
• Inspect the seal stack, including O-rings, lip seals, and backup rings
  
• Replace all seals using OEM or high-quality aftermarket kits
  
• Reassemble with torque specs and apply thread sealant where required
  
• Refill gear oil and bleed hydraulic lines if necessary
  

Some technicians recommend replacing both sides if one seal fails, especially on older machines. This ensures balanced performance and avoids future downtime.
Checking for Hose Integrity and External Factors
Before assuming internal failure, it’s essential to inspect:

• Hydraulic hoses leading to the final drive motor
  
• Fittings and banjo bolts for cracks or looseness
  
• Swivel joints and quick couplers for wear
  
• Track frame welds and mounting bolts for stress fractures
  

In one case, a contractor discovered a hairline crack in a banjo fitting that mimicked seal failure. Replacing the fitting resolved the leak without opening the motor.
Seal Kit Sourcing and Compatibility Notes
Kubota does not always list final drive seal kits separately in older manuals. Strategies for sourcing include:

• Cross-referencing motor part numbers with OEM suppliers like Nachi or Kayaba
  
• Measuring seal dimensions and matching with hydraulic seal catalogs
  
• Consulting compact excavator rebuild shops for kit bundles
  
• Verifying compatibility with U35-3 or U35-4 models if motor design is shared
  

Seal kits typically include:

• Outer dust seal
  
• Inner hydraulic seal
  
• O-rings and backup rings
  
• Retaining clips or snap rings
  

Prices range from $80 to $250 depending on source and completeness.
A Story from the Field
In 2023, a landscaper in Mississippi noticed fluid pooling beneath his U35S2 after a week of heavy grading. Suspecting a hose leak, he cleaned the area and ran the machine briefly. The leak persisted, and closer inspection revealed fluid seeping from the rear motor housing. With no manual available, he removed the cover using an Allen wrench set and found a torn lip seal. After sourcing a replacement from a hydraulic shop and reassembling the drive, the machine returned to service compacting gravel for a patio installation.
Preventive Measures and Long-Term Reliability
To extend final drive life:

• Inspect seals annually, especially after winter storage
  
• Avoid high-pressure washing near motor housings
  
• Use correct hydraulic fluid and gear oil grades
  
• Monitor track tension to reduce side loading on seals
  
• Replace worn sprockets and bearings to prevent vibration
  

Adding a magnetic drain plug to the gear case can help detect early wear by capturing metal particles.
Conclusion
Hydraulic leaks from the final drive of the Kubota U35S2 often point to seal failure within the motor housing. With careful disassembly, seal replacement, and hose inspection, the issue can be resolved without full motor rebuild. Though documentation may be sparse, mechanical intuition and methodical repair restore the compact excavator’s mobility—keeping it ready for the next trench, slope, or foundation cut.