Does a Bobcat Hydraulic System Require Priming

[MikePhua]

The Importance of Priming in Compact Hydraulic Systems
Priming a hydraulic system is the process of removing trapped air and ensuring fluid reaches all components before full operation. In compact equipment like Bobcat skid steers and mini excavators, priming is especially important after major service events such as hose replacement, cylinder rebuilds, or complete fluid changes. Without proper priming, the system may suffer from cavitation, erratic movement, or pump damage.
Bobcat, founded in 1947 and now part of the Doosan Group, has produced millions of compact machines globally. Its hydraulic systems are known for precision control and high flow-to-weight ratios, but they rely on clean, air-free fluid to perform reliably.
What Happens If You Skip Priming
Failure to prime a hydraulic system can lead to:

• Cavitation: Vapor bubbles form in the pump due to low pressure, causing metal erosion and noise.
  
• Air Locks: Trapped air prevents fluid from reaching actuators, resulting in non-responsive controls.
  
• Pressure Fluctuations: Inconsistent flow leads to jerky movement and poor attachment performance.
  
• Seal Damage: Air pockets can cause pressure spikes that rupture seals or damage valve seats.
  
• Pump Overload: Dry running increases friction and heat, shortening pump life.
  

In 2022, a contractor in Colorado replaced a lift cylinder on a Bobcat T650 without priming the system. The machine exhibited slow boom response and a high-pitched whine. After cycling the controls and bleeding the lines, performance returned to normal—but not before the pump showed signs of premature wear.
Steps to Prime a Bobcat Hydraulic System
Priming procedures vary slightly by model, but the general approach includes:

• Fill the Reservoir
  Use the correct hydraulic fluid and fill to the recommended level. Bobcat typically specifies ISO 46 or ISO 32 depending on climate.
  
• Cycle the Controls Slowly
  With the engine running at low idle, move each hydraulic function through its full range. This draws fluid into the cylinders and displaces air.
  
• Bleed High Points
  Loosen fittings at the top of cylinders or valve blocks to release trapped air. Retighten once fluid appears.
  
• Check for Leaks
  Inspect all connections and hoses during priming. Air ingress often comes from loose fittings or cracked seals.
  
• Monitor Fluid Level
  As air escapes, fluid may drop. Refill as needed to maintain proper level.
  
• Use a Load if Needed
  Applying light resistance (such as lifting a small object) can help force fluid through the system and purge air faster.
  

Terminology notes:

• Hydraulic Reservoir: The tank that stores fluid and allows for thermal expansion.
  
• Pilot Control: A low-pressure signal system that actuates main valves.
  
• Relief Valve: A safety device that limits system pressure.
  
• Return Line: The path fluid takes back to the reservoir after completing work.
  
• Charge Pressure: The baseline pressure that keeps fluid circulating and prevents cavitation.
  

Tips for Efficient Priming and Long-Term Reliability
To improve priming and reduce future issues:

• Always retract cylinders before draining fluid. This minimizes air volume.
  
• Use clean, filtered fluid to avoid introducing contaminants.
  
• Replace filters during fluid changes to prevent flow restriction.
  
• Label hydraulic lines for easier troubleshooting.
  
• Install a sight gauge or fluid level sensor for quick checks.
  

Recommended upgrades:

• Add quick-bleed valves at cylinder ports
  
• Use transparent return lines for visual confirmation of flow
  
• Install a pressure gauge on the pump outlet
  
• Retrofit with a fluid temperature sensor to monitor heat buildup
  
• Use synthetic hydraulic oil for better cold-start performance
  

Conclusion
Yes, Bobcat hydraulic systems do require priming—especially after service or fluid replacement. Proper priming ensures smooth operation, protects components, and extends system life. Whether you're running a compact loader in tight quarters or a mini excavator in trenching work, taking the time to prime the system is a small step that prevents big problems. In the world of hydraulics, air is the enemy—and fluid flow is everything.