Troubleshooting Hydraulic Lift Failure on a 1986 Ford F800

[MikePhua]

The F800 and Its Underbody Hoist System
The Ford F800, part of the medium-duty truck lineup produced from the 1980s through the early 2000s, was widely used for dump bodies, utility beds, and vocational applications. The 1986 model often featured a hydraulic underbody hoist system powered by a PTO-driven pump mounted to the transmission. Unlike conventional setups with external reservoirs, this system uses the lift cylinder itself as the hydraulic reservoir, creating a closed-loop configuration that can be difficult to prime after service.
These trucks were built for simplicity and durability, but their hydraulic systems—especially when modified or aged—can present unique challenges when air enters the circuit or components are disturbed during unrelated repairs.
Symptoms and Initial Observations
After replacing the bellhousing gasket, the operator reconnected the PTO shaft and verified engagement. However, the dump box refused to lift. The pump had two input shafts—clockwise and counterclockwise—and the correct shaft was confirmed. Both rotated properly, yet the system produced no lift and sounded dry.
Despite topping off the fluid, pulling the suction line, inspecting the valve spool, and checking for oil at multiple points, the system remained unresponsive. The pump and valve were disassembled and found to be mechanically sound. The issue appeared to be air entrapment or loss of prime.
Terminology Notes

• PTO (Power Take-Off): A mechanical interface that transfers engine power to auxiliary equipment like hydraulic pumps.
  
• Closed Hydraulic System: A configuration where the fluid reservoir is integrated into the cylinder or actuator, limiting external access.
  
• Priming: The process of removing air from a hydraulic pump to restore fluid flow.
  
• Valve Spool: A sliding component inside a hydraulic valve that directs fluid to different ports.
  

Diagnosis and Field Solutions
The system showed signs of air lock—a common issue in gear-type hydraulic pumps, which cannot self-prime if dry. Several strategies were attempted:

• Running the pump with the fill port open to vent trapped air
  
• Cracking pressure lines to check for flow
  
• Applying air pressure to the reservoir to force fluid into the pump
  
• Manually turning the pump shaft while bleeding lines
  

Eventually, through persistent bleeding and manipulation of the valve, the system began to move oil. The pump was reassembled with new seals, and the lift function was restored.
Common Causes of Air Entrapment

• Disconnected or disturbed suction lines during transmission work
  
• Worn input shaft seals allowing air ingress without visible leaks
  
• Improper reassembly of pump components
  
• Extended downtime, allowing fluid to drain back and air to enter
  

One mechanic noted that gear pumps are especially sensitive to dry starts. Even a small amount of air can prevent the pump from generating suction, requiring manual priming or external pressure to restore function.
Recommendations for Future Maintenance

• Always pre-fill suction lines before reconnecting the pump
  
• Use air pressure cautiously to assist priming—no more than 5 psi
  
• Replace input shaft seals if suspected, even without visible leaks
  
• Cycle the valve manually while bleeding lines to encourage flow
  
• Document pump orientation and shaft engagement during disassembly
  

Conclusion
Hydraulic lift failure on a Ford F800 with an underbody hoist system is often caused by air lock following service. With careful priming, seal replacement, and line bleeding, the system can be restored without replacing major components. Understanding the behavior of gear-type pumps and closed hydraulic circuits is essential for diagnosing and resolving these issues efficiently.