Why Hydraulic Tanks Sometimes Fail to Hold Pressure

[MikePhua]

Hydraulic Tank Pressurization and Its Role in Excavator Performance
Modern excavators like the Volvo EC210 rely on pressurized hydraulic tanks to maintain consistent fluid flow and prevent cavitation. The tank’s positive pressure—typically around 14 psi—is generated internally by the displacement of hydraulic cylinders during full stroke cycles. This pressure ensures that the hydraulic pump receives fluid without air intrusion, especially during high-demand operations like boom-down or bucket curl.
Volvo Construction Equipment, founded in 1832 and headquartered in Sweden, has long emphasized reliability and operator comfort. The EC210 series became one of its best-selling models in the 20-ton class, widely used in road building, quarrying, and utility trenching. Its hydraulic system includes a breather assembly, pressure cap, and return filters—all critical to maintaining tank integrity.
Symptoms of Inconsistent Tank Pressure
Operators may notice that the hydraulic tank sometimes holds pressure and other times does not. This inconsistency often manifests as:

• Jerky boom-down movement
  
• Audible absence of air release when checking the tank cap
  
• Pressure present during cold starts but absent after extended operation
  

These symptoms suggest a failure in the tank’s ability to retain residual pressure, which can lead to sluggish hydraulics and increased wear on the pump.
Breather Filter and Pressure Cap Issues
One common culprit is the breather filter assembly. If recently replaced, the new filter may have a different flow rating or internal restriction. Even if it visually matches the original, subtle differences in design can affect pressure retention.
The pressure cap itself is another frequent failure point. It contains a spring-loaded valve and sealing gasket that must maintain a tight seal under operating conditions. If the gasket is worn or the spring fatigued, the cap may vent prematurely or fail to hold pressure altogether.
Terminology Notes

• Cavitation: The formation of air bubbles in hydraulic fluid due to low pressure, which can damage pumps and valves.
  
• Breather Filter: A component that allows air to enter or exit the hydraulic tank while filtering contaminants.
  
• Pressure Cap: A sealed cap that maintains internal tank pressure and prevents fluid loss.
  
• Residual Pressure: The remaining pressure in a closed system after active operation has ceased.
  

Testing and Diagnosis
To confirm the issue:

• Cycle all hydraulic functions to full stroke while the oil is cold. This helps charge the tank with pressure.
  
• Listen for air release when loosening the cap briefly. A hiss indicates positive pressure.
  
• Swap the breather filter with one from a known-good machine to isolate the fault.
  
• Inspect the pressure cap for cracks, gasket wear, or spring failure. Replacement caps may cost $150 or more, but are widely shared across excavator models.
  

If pressure is present during startup but disappears after an hour of operation, thermal expansion or internal leakage may be affecting the cap’s seal. In such cases, replacing the cap is often the most effective solution.
Field Anecdotes and Practical Advice
One operator in Canada noticed erratic boom behavior after replacing his breather filter. After swapping it with the filter from his mini excavator, the issue persisted. Only after replacing the pressure cap did the tank consistently hold pressure.
Another technician in the UK emphasized that fully stroking the arm and bucket during cold starts helps build tank pressure naturally. He recommended performing this cycle daily before heavy digging to ensure optimal hydraulic response.
Recommendations for Maintenance

• Replace breather filters annually or after 1,000 hours of operation
  
• Inspect pressure caps quarterly, especially after cold weather cycles
  
• Use OEM parts for breather assemblies and caps to ensure compatibility
  
• Monitor hydraulic fluid levels and temperature to detect early signs of cavitation
  

If jerky hydraulics persist, consider testing in “F” mode or diagnostic mode to isolate valve or pump irregularities.
Conclusion
Hydraulic tank pressure is essential for smooth excavator operation, and its loss can stem from subtle issues in the breather filter or pressure cap. By understanding the system’s self-pressurizing nature and inspecting key components, operators can restore consistent performance and prevent long-term damage. Even small parts like a cap gasket can make a big difference in hydraulic reliability.