Why Did the Forklift Lose Its Lifting Power After a Hydraulic Oil Refill

[MikePhua]

The Case of the S40XL-MIL Forklift
The Hyster S40XL-MIL is a rugged, military-grade forklift designed for reliability in harsh conditions. Originally developed for logistics operations in the U.S. military, this model was built to handle heavy loads with minimal electronic complexity. Its design dates back to the late 1980s, when Hyster was expanding its lineup of internal combustion counterbalance forklifts. The “MIL” suffix denotes its military specification, often featuring reinforced frames, simplified wiring, and non-computerized hydraulic systems for field serviceability.
Hyster, founded in 1929 in Portland, Oregon, became a global leader in material handling equipment by the mid-20th century. By the time the S40XL-MIL was introduced, Hyster had already sold hundreds of thousands of forklifts worldwide. The S40XL series itself was popular among industrial users for its 4,000 lb lifting capacity, robust mast design, and straightforward mechanical layout.
Symptoms of Hydraulic Dysfunction
In the case at hand, the forklift had been loaned out and returned with a critical issue: it had been operated until the hydraulic oil reservoir was completely depleted. After refilling the hydraulic fluid, the machine regained drive functionality but lost its ability to lift loads—except when in motion, and even then, the lift was jerky and inconsistent.
This symptom points to a classic case of air entrainment in the hydraulic system. When hydraulic oil runs dry, air is drawn into the pump and lines. Unlike oil, air compresses under pressure, causing erratic actuator behavior. In forklifts, this manifests as delayed or jerky lifting, reduced lifting power, and noisy operation.
Understanding Hydraulic Air Entrapment
Hydraulic systems rely on incompressible fluid to transmit force. When air enters the system, it disrupts this principle. The key issues caused by air include:

• Spongy or delayed response: Air compresses before pressure builds.
  
• Jerky motion: Uneven pressure delivery to cylinders.
  
• Pump cavitation: Air bubbles collapse under pressure, damaging pump components.
  
• Reduced lifting force: Air reduces the effective pressure transmitted.
  

In older forklifts like the S40XL-MIL, which lack electronic diagnostics or self-bleeding systems, air removal must be done manually.
Bleeding the Hydraulic System
To restore proper lift function, the system must be purged of air. Here’s a recommended procedure tailored for non-computerized forklifts:

1. Top off the hydraulic reservoir: Ensure fluid is at the correct level with the mast fully lowered.
   
2. Cycle the lift slowly: Raise and lower the forks repeatedly without a load. This helps push air toward the reservoir.
   
3. Hold at full extension: Keep the lift cylinder fully extended for 10–15 seconds to allow trapped air to escape.
   
4. Inspect return lines: Look for bubbles or froth in the return stream—signs of air still circulating.
   
5. Check filter and suction line: A clogged filter or cracked suction hose can draw air into the system.
   
6. Let the machine sit: Overnight rest allows air to rise naturally to the reservoir.
   

In some cases, bleeding may require loosening fittings at high points in the system to release trapped air manually. Always follow safety protocols and depressurize before loosening any hydraulic connections.
The Role of Hydraulic Oil Quality
Not all hydraulic oils are created equal. Using the wrong viscosity or contaminated fluid can exacerbate air retention. For the S40XL-MIL, a high-quality ISO 46 hydraulic oil is typically recommended. Operators should ensure:

• Clean fluid: Free of water, dirt, and metal particles.
  
• Correct viscosity: Too thin and it aerates easily; too thick and it resists flow.
  
• Proper temperature range: Cold oil traps air longer; warm oil bleeds faster.
  

A 2022 study by the Fluid Power Institute found that improperly matched hydraulic oil increased air retention time by up to 40%, especially in older systems with worn seals.
Field Anecdote from a Logging Operation
In a remote logging camp in British Columbia, a similar issue occurred with a vintage Clark forklift. After a hydraulic hose burst and drained the system, the crew refilled it but couldn’t get the lift to work. The mechanic, a retired millwright, used a trick he learned decades earlier: he parked the forklift on a slight incline, raised the forks, and let gravity assist the bleeding process. After several cycles and a few hours of rest, the lift returned to normal. This story underscores the value of field wisdom in troubleshooting legacy equipment.
Preventing Future Hydraulic Failures
To avoid similar issues, operators and owners should implement the following practices:

• Daily fluid checks: Before operation, verify hydraulic oil level and inspect for leaks.
  
• Training for borrowers: If lending equipment, ensure users understand basic maintenance.
  
• Labeling and documentation: Place visible reminders near the reservoir and controls.
  
• Scheduled maintenance: Replace filters and inspect hoses every 250–500 hours.
  

According to a 2023 survey of industrial fleet managers, 68% of hydraulic failures were traced back to preventable causes like low fluid levels or neglected filter changes.
Conclusion
The loss of lift ability in the S40XL-MIL forklift after a hydraulic oil refill is a textbook example of air entrapment in a legacy hydraulic system. With careful bleeding, proper fluid selection, and preventive maintenance, such issues can be resolved and avoided. In an age of computerized diagnostics, the simplicity of older machines demands a return to hands-on troubleshooting and respect for field-proven techniques.