Rear Spool Valve Seal Issues on the Bobcat 773 Skid Steer Loader

[MikePhua]

The Bobcat 773 and Its Hydraulic Architecture
The Bobcat 773 was introduced in the late 1990s as part of Bobcat’s G-series skid steer lineup. Manufactured by Melroe Company, which later became part of Doosan and now Bobcat Company under Hyundai, the 773 quickly gained popularity for its balance of power, compact footprint, and hydraulic versatility. With over 100,000 units sold globally, it remains a staple in landscaping, construction, and agricultural fleets.
The 773 features a hydrostatic drive system and a robust hydraulic control valve assembly located beneath the operator’s seat. This valve block governs lift, tilt, and auxiliary functions, using spool valves actuated by mechanical or pilot controls. Each spool is sealed with O-rings and backup rings to prevent internal leakage and maintain pressure integrity.
Understanding Spool Valve Seal Function
Spool valves operate by sliding within precision-machined bores, directing hydraulic fluid to specific circuits. Seals around the spool prevent cross-port leakage and external seepage. These seals are typically:

• Buna-N or Viton O-rings
  
• Teflon backup rings
  
• U-cup seals in high-pressure applications
  

In the Bobcat 773, the rear spool valve—often responsible for auxiliary hydraulic functions—can develop leaks due to seal degradation, contamination, or improper installation.
Symptoms of Seal Failure
Operators may notice:

• Hydraulic fluid leaking from the rear of the valve block
  
• Loss of pressure in auxiliary circuits
  
• Erratic attachment behavior
  
• Visible oil pooling beneath the loader arms or valve housing
  
• Increased effort required to actuate controls
  

In one case from a snow removal crew in Michigan, a 773 began losing hydraulic power to its snow blower attachment. Inspection revealed a torn O-ring on the rear spool valve, allowing fluid to bypass internally and leak externally.
Causes of Seal Damage
Common contributors to seal failure include:

• Age-related hardening or cracking
  
• Contaminants such as metal shavings or dirt
  
• Over-tightened retaining caps distorting seal geometry
  
• Incorrect seal type or size during previous service
  
• Excessive heat from prolonged high-pressure operation
  

Hydraulic fluid contaminated with water or incompatible additives can accelerate seal degradation. Using the wrong fluid viscosity also affects seal performance, especially in cold climates.
Disassembly and Seal Replacement Procedure
To service the rear spool valve:

• Relieve hydraulic pressure and disconnect the battery
  
• Remove the operator seat and access panel
  
• Clean the valve block exterior to prevent contamination
  
• Remove retaining bolts and carefully extract the spool
  
• Inspect the bore for scoring or corrosion
  
• Replace all seals with OEM-spec kits, including O-rings and backup rings
  
• Lubricate seals with hydraulic fluid before installation
  
• Reassemble and torque bolts evenly to factory spec
  

Some technicians use a seal pick and soft-jaw pliers to avoid damaging the spool or bore. Always verify seal orientation, especially for backup rings, which must support the O-ring against pressure.
Field Anecdote and Practical Insight
In Alberta, a contractor servicing his 773 for forestry work noticed hydraulic fluid dripping near the rear valve block. After removing the spool, he found that the previous repair had used a generic O-ring without a backup ring. The seal had extruded under pressure, causing internal bypass. He replaced the seal with a Bobcat kit and added a magnetic drain plug to monitor future wear. The loader returned to full function within hours.
Preventive Measures and Long-Term Reliability
To extend seal life and prevent future leaks:

• Use only manufacturer-approved hydraulic fluid
  
• Change fluid and filters every 500 hours or annually
  
• Install inline filters on auxiliary circuits to catch debris
  
• Avoid prolonged idling with attachments under load
  
• Monitor control response and inspect for early signs of leakage
  

Some operators retrofit their machines with pressure gauges on auxiliary lines to detect drops in real time. Others install quick-connect dust caps to prevent contamination during attachment changes.
Conclusion
Rear spool valve seal failure on the Bobcat 773 is a manageable issue when addressed promptly and correctly. By understanding the hydraulic system’s design, using proper seal kits, and maintaining fluid cleanliness, operators can restore full functionality and avoid costly downtime. The 773 remains a reliable workhorse, and with careful attention to its hydraulic internals, it continues to perform across seasons and job sites.