CAT 955L Hydraulic System Troubleshooting and Restoration

[MikePhua]

The CAT 955L and Its Role in Mid-Size Track Loader Operations
The Caterpillar 955L track loader was introduced in the 1970s as part of CAT’s evolution in crawler loader design. Positioned between lighter models like the 941 and heavier machines like the 977, the 955L offered a balance of power, maneuverability, and lifting capacity. With an operating weight around 32,000 pounds and a bucket capacity exceeding 2 cubic yards, it became a staple in excavation, demolition, and site preparation.
Caterpillar, founded in 1925, had by then refined its hydraulic systems to deliver consistent performance under load. The 955L featured a robust hydraulic architecture, integrating gear-driven pumps, spool valves, and double-acting cylinders. Its mechanical simplicity made it popular among operators who valued field serviceability over electronic complexity.
Hydraulic System Configuration and Key Components
The hydraulic system in the 955L is open-center and gear-pump driven. It powers the lift arms, bucket tilt, and optional auxiliary functions. Core components include:

• Gear-type hydraulic pump mounted to the engine
  
• Control valve block with mechanical linkages
  
• Lift and tilt cylinders with dual ports
  
• Hydraulic reservoir with breather and sight gauge
  
• Return filter and suction strainer
  
• Steel lines and flexible hoses routed through the loader frame
  

The system operates at pressures around 2,500 psi and flows approximately 40 gallons per minute. The pump is driven directly off the engine, and fluid returns to the tank via a filter to remove contaminants.
Common Hydraulic Problems and Their Symptoms
Operators may encounter several hydraulic issues over time, especially in high-hour machines:

• Slow or weak lift and tilt response
  
• Jerky or uneven cylinder movement
  
• Hydraulic fluid overheating
  
• Leaks at fittings, hoses, or cylinder seals
  
• Noisy pump or cavitation sounds
  
• Contaminated fluid with metal particles or discoloration
  

Typical causes include:

• Worn pump gears reducing flow and pressure
  
• Internal cylinder leakage bypassing seals
  
• Blocked suction strainer or clogged return filter
  
• Air ingress from cracked hoses or loose fittings
  
• Valve spool wear causing erratic control
  
• Low fluid level or incorrect viscosity
  

In one case from Alberta, a 955L began showing sluggish bucket response. After checking fluid levels and inspecting hoses, the technician discovered a partially collapsed suction hose and a clogged strainer. Replacing both restored full hydraulic function.
Diagnostic Procedures and Field Testing
Effective troubleshooting involves both visual inspection and pressure testing. Recommended steps include:

• Check fluid level and condition in the reservoir
  
• Inspect hoses and fittings for leaks or abrasion
  
• Remove and clean suction strainer and return filter
  
• Use a pressure gauge at test ports to verify pump output
  
• Cycle cylinders and observe for drift or hesitation
  
• Listen for pump noise indicating cavitation or aeration
  

In a rebuild project in Texas, a 955L showed erratic lift behavior. Pressure testing revealed low output at the pump. The issue was traced to worn gear teeth and a cracked pump housing. After installing a remanufactured pump and flushing the system, the loader returned to full productivity.
Repair Strategies and Component Replacement
When hydraulic failure is confirmed, repair options include:

• Rebuilding or replacing the hydraulic pump
  
• Resealing lift and tilt cylinders
  
• Replacing worn valve spools and springs
  
• Installing new hoses and fittings
  
• Flushing the system and replacing fluid
  
• Upgrading filters to higher-efficiency models
  

Recommended fluids:

• Hydraulic oil: ISO 46 or ISO 68 depending on climate
  
• Filter: Replace every 500 hours or sooner in dusty conditions
  
• Reservoir: Clean annually and inspect breather cap
  

In British Columbia, a contractor rebuilt the hydraulic system on a 955L after 10,000 hours. The overhaul included new cylinders, hoses, and a modern spin-on filter conversion. The machine showed smoother control and reduced operating temperature.
Preventive Maintenance and Long-Term Reliability
To extend hydraulic system life:

• Inspect hoses and fittings monthly
  
• Replace filters at recommended intervals
  
• Monitor fluid temperature and avoid prolonged idling
  
• Keep reservoir sealed and breather clean
  
• Use magnetic drain plugs to detect wear particles
  
• Train operators to avoid abrupt control movements under load
  

In Georgia, a fleet manager implemented a hydraulic inspection checklist for all track loaders. By catching leaks and low fluid levels early, they reduced downtime and extended component life by 30%.
Operator Stories and Practical Wisdom
In Maine, a 955L used for demolition began leaking fluid near the control valve. The operator traced the issue to a cracked hard line and replaced it with a flexible hose. The loader resumed work without further leaks.
In New Zealand, a forestry crew noticed overheating during long pushes. The hydraulic cooler was partially blocked with bark debris. After cleaning and installing a mesh guard, the machine operated within normal temperature range.
Conclusion
The hydraulic system in the CAT 955L is a durable, field-serviceable design built for demanding environments. With proper diagnostics, timely repairs, and preventive care, it can deliver thousands of hours of reliable performance. Whether lifting debris or shaping terrain, the health of the hydraulics determines the precision and power of the job. In the world of track loaders, fluid pressure is the muscle—and clean, consistent flow is the key to keeping that muscle strong.