Diagnosing Hydraulic Priority Issues in the Dresser 515C Loader

[MikePhua]

The Dresser 515C and Its Industrial Footprint
The Dresser 515C wheel loader was part of a transitional era in heavy equipment manufacturing. Originally produced under the Dresser brand—a legacy of International Harvester’s construction division—the 515C was built during the mid-1990s, just before Dresser’s merger with Komatsu reshaped its product line. Known for its simplicity and reliability, the 515C was often used in snow removal, light aggregate handling, and municipal work.
Equipped with a gear-type hydraulic pump and mechanical linkages for loader control, the 515C was designed to be serviceable in the field with minimal electronics. Its operating weight hovered around 24,000 lbs, and it featured a Cummins diesel engine producing roughly 125 horsepower. While exact production numbers are scarce, the 515C was widely distributed across North America, especially in colder regions where its enclosed cab and robust heating system were appreciated.

The Symptom Steering Away from Loader Function
One of the more perplexing issues reported with the 515C involves a loss of loader function when the steering wheel is turned—even slightly. In normal operation, the loader arms and bucket respond smoothly to control inputs. But when steering is engaged, hydraulic flow to the loader drops dramatically, requiring the operator to rev the engine to regain movement.
This behavior points to a hydraulic flow prioritization problem. In systems where a single pump feeds multiple functions—such as steering and loader operation—a priority valve is used to ensure steering always receives adequate flow. If this valve malfunctions, it can starve the loader circuit even under light steering demand.

Understanding the Priority Valve System
A priority valve is a hydraulic component that directs flow to essential functions first—in this case, steering. Once steering demand is satisfied, excess flow is diverted to secondary functions like the loader. In the Dresser 515C, this valve is integrated into the steering valve block beneath the cab.
Key components of the system include:

• Gear-type hydraulic pump
  
• Steering valve block with integrated priority valve
  
• Loader control valve
  
• Solid rod linkages for mechanical actuation
  
• Hydraulic reservoir and return lines
  

If the priority valve spool becomes stuck, worn, or contaminated, it may continuously divert flow to steering—even when no input is given. This results in loader sluggishness or complete failure unless engine RPM is increased to compensate.

Common Causes and Field Diagnoses
Several factors can contribute to priority valve malfunction:

• Contaminated hydraulic fluid due to aging or improper filtration
  
• Incorrect filter micron rating causing flow restriction
  
• Air trapped in the hydraulic system after fluid changes
  
• Mechanical wear in the valve spool or housing
  
• Misalignment of control rods or linkage interference
  

In one case, a 515C purchased from a defunct dealership had only 1,300 original hours and was used exclusively for snow removal. After a full fluid and filter change, the loader began exhibiting the steering-related hydraulic dropout. Despite the pump appearing “like new” internally, the issue persisted until the priority valve was inspected.
A technician in Michigan recalled a similar issue on a larger Dresser loader, where a stuck spool in the steering valve block caused intermittent loader failure. After disassembling the valve and cleaning the internal passages, the machine returned to normal operation.

Switch Pump Confusion and Hydraulic Architecture
Some operators mistakenly believe the 515C uses multiple hydraulic pumps—one for steering, one for loader, and one for transmission. In reality, the machine uses a single gear pump, with flow divided by internal valve logic. The term “switch pump” refers to the behavior of the priority valve, which “switches” flow between steering and loader based on demand.
The transmission uses a separate charge pump, and the loader controls are actuated via solid rods rather than cables, reducing the chance of mechanical slop or miscommunication. The hydraulic block under the cab serves as the central distribution point for both steering and loader functions.

Solutions and Preventive Measures
To resolve the issue, the following steps are recommended:

• Inspect and clean the priority valve inside the steering valve block
  
• Replace hydraulic fluid with manufacturer-recommended grade
  
• Verify filter specifications—use correct micron rating to avoid restriction
  
• Bleed the hydraulic system to remove trapped air
  
• Check mechanical linkages for binding or misalignment
  
• Review the hydraulic schematic to trace flow paths and valve locations
  

If the priority valve is damaged beyond repair, replacement may be necessary. Always consult the parts diagram and confirm item numbers—such as components 48, 49, and 50 in the steering valve assembly.
Preventive maintenance tips include:

• Changing hydraulic fluid every 1,000 hours or annually
  
• Replacing filters every 500 hours
  
• Keeping the hydraulic tank clean and sealed
  
• Monitoring steering and loader responsiveness during startup
  

Conclusion
The Dresser 515C remains a dependable machine when its hydraulic system is properly maintained. Issues like loader dropout during steering are often traced to the priority valve—a small but critical component that governs flow distribution. By understanding the interplay between steering and loader circuits, and applying methodical diagnostics, operators can restore full functionality and extend the life of this classic loader. Whether clearing snow in Massachusetts or moving gravel in Alberta, the 515C continues to earn its place in the field through rugged simplicity and smart design.