Rebuilding the Travel Pump on a CAT D5C Hystat Dozer

[MikePhua]

The CAT D5C Hystat and Its Hydraulic Architecture
The Caterpillar D5C Hystat dozer, introduced in the late 1990s, marked a shift in compact dozer design by integrating hydrostatic drive technology. Unlike traditional torque converter systems, the Hystat configuration uses dual variable-displacement axial piston pumps to control each track independently, allowing for precise maneuvering, smooth directional changes, and infinite speed variability.
These pumps—often identified as Rexroth A4VG90 units—are mounted side-by-side and feed hydraulic motors directly. The system eliminates the need for clutches or gearboxes, relying instead on fluid pressure and swash plate angle to modulate speed and torque. Caterpillar adopted this design to improve operator control and reduce mechanical complexity, especially in grading and finish work.
Symptoms of Seal Failure and Air Infiltration
Over time, the travel pumps may develop external leaks or internal air infiltration due to degraded O-rings, gaskets, or mating surface wear. Common symptoms include:

• Visible hydraulic fluid seepage around pump housings
  
• Whining or cavitation noise during travel
  
• Hesitation or jerky movement when changing direction
  
• Reduced pushing power under load
  

Terminology clarification:
- Cavitation: The formation of air bubbles in hydraulic fluid due to low pressure or vacuum conditions
- Lapping: A precision surface finishing process that removes high spots and restores flatness
- Swash Plate: An angled plate inside the pump that controls piston stroke and thus fluid displacement
- Case Drain: A low-pressure return line that carries leakage oil from the pump housing back to the reservoir
In one case, the operator suspected that aged seals were allowing air into the system, causing erratic travel behavior. The pumps were still functional, but the performance degradation and fluid loss warranted a rebuild.
Is a DIY Rebuild Feasible
Rebuilding a Rexroth A4VG90 pump is technically demanding but achievable for skilled mechanics with access to clean workspace, precision tools, and component manuals. The process involves:

• Removing the pump from the machine and draining residual fluid
  
• Disassembling the housing and separating the rotating group
  
• Cleaning and lapping mating surfaces to restore sealing integrity
  
• Replacing all O-rings, gaskets, and wear plates with OEM-grade parts
  
• Reassembling with proper torque specs and cleanliness standards
  

Challenges include:

• Handling delicate components like pistons, slipper pads, and valve plates
  
• Maintaining alignment of the swash plate and control linkage
  
• Avoiding contamination during reassembly
  
• Pressure testing the rebuilt pump before installation
  

A technician in Texas shared that he rebuilt both pumps on a D5C using only the component manual and a homemade lapping plate. He emphasized the importance of cleanliness and patience, noting that even a single grain of dirt could compromise the rebuild.
When to Involve a Hydraulic Shop
If the pump shows signs of internal scoring, bearing failure, or swash plate damage, a professional hydraulic shop may be the better option. Shops have access to:

• Precision lapping machines
  
• Flow benches for performance testing
  
• OEM calibration tools for control valves
  
• Clean rooms for contamination-free assembly
  

Costs vary, but a full rebuild typically ranges from $2,500 to $4,000 per pump, depending on labor and parts. Some shops offer exchange programs or refurbished units with warranty.
Reinstallation and System Flushing
After rebuilding, the pump must be reinstalled carefully:

• Align mounting flanges and torque bolts evenly
  
• Reconnect case drain and pressure lines with new seals
  
• Prime the system with clean hydraulic fluid
  
• Bleed air from the circuit using manual cycling or vacuum fill
  
• Monitor for leaks and verify travel response under load
  

It’s critical to flush the entire hydraulic system before startup to remove debris and prevent contamination. Replace filters and inspect reservoir screens for metal or seal fragments.
Preventive Maintenance and Longevity Tips
To extend pump life and avoid future rebuilds:

• Change hydraulic fluid every 1,000 hours or annually
  
• Use high-quality fluid with anti-foam and anti-wear additives
  
• Inspect case drain flow for early signs of internal leakage
  
• Monitor travel response and noise for changes
  
• Keep the reservoir sealed and vented properly
  

Operators should also avoid abrupt directional changes at high speed, which can stress the swash plate and control linkage. Gradual modulation improves longevity and reduces shock loading.
Conclusion
Rebuilding the travel pump on a CAT D5C Hystat is a precision task that demands attention to detail, cleanliness, and mechanical discipline. Whether done in-house or by a hydraulic specialist, restoring the pump’s sealing surfaces and internal geometry can bring the machine back to full performance.
The Hystat system, while complex, offers unmatched control and responsiveness when properly maintained. And for those willing to dive into the heart of hydrostatic drive, the reward is not just a leak-free pump—but a deeper understanding of the fluid power that moves machines and reshapes terrain.