Diagnosing Boom Drift on the Volvo MC135 Skid Steer

[MikePhua]

The Volvo MC135 and Its Hydraulic Architecture
The Volvo MC135 skid steer loader was introduced in the mid-2000s as part of Volvo Construction Equipment’s push into compact machinery. Built for versatility in urban construction, landscaping, and light material handling, the MC135 featured a vertical lift path, robust frame, and a hydraulic system designed for precision and durability. With an operating capacity of approximately 1,350 pounds and powered by a 50–60 HP diesel engine, the MC135 competed with models from Bobcat, Case, and New Holland in the mid-range skid steer market.
Volvo’s hydraulic design emphasized simplicity and serviceability, using pilot-operated valves and load-holding features to maintain boom position under load. However, like many compact loaders, the MC135 is susceptible to boom drift—where the lift arms slowly lower over time, even when the controls are neutral and the engine is off.
Terminology Annotation

• Boom Drift: The unintended lowering of the lift arms due to internal hydraulic leakage or valve failure.
  
• Lift Cylinder: A double-acting hydraulic actuator responsible for raising and lowering the boom.
  
• Control Valve Spool: A sliding component inside the valve block that directs fluid flow based on joystick input.
  
• Load-Holding Check Valve: A valve designed to prevent fluid from escaping the cylinder when the control is in neutral.
  

Symptoms and Initial Observations
Operators may notice:

• Boom slowly lowers when parked, even with engine off.
  
• No visible external leaks.
  
• Bucket remains level while arms drift downward.
  
• Drift occurs faster when machine is warm.
  

These symptoms suggest internal leakage—either within the lift cylinders or the control valve block. External leaks would leave visible oil traces, while internal bypassing allows fluid to escape from the cylinder’s pressure chamber back to the tank or through the valve body.
Cylinder Seal Wear and Internal Bypass
Lift cylinders contain piston seals, wear bands, and rod seals. Over time, these components degrade due to heat, contamination, and pressure cycling. When the piston seal fails, fluid bypasses from one side of the piston to the other, causing the boom to settle.
To test for internal bypass:

• Raise the boom fully and shut off the engine.
  
• Disconnect the hydraulic lines from the lift cylinder ports.
  
• Cap the lines and observe whether fluid escapes from the cylinder ports.
  
• If fluid flows from the pressure side without actuation, the piston seal is compromised.
  

In one case from a contractor in Alberta, both lift cylinders on an MC135 were rebuilt after boom drift worsened during winter. The seals had hardened and cracked, allowing rapid bypass under load.
Valve Block and Spool Leakage
If the cylinders test clean, the next suspect is the control valve block. Spool valves can leak internally due to scoring, contamination, or worn O-rings. Load-holding check valves may also fail to seat properly, especially if debris or varnish builds up.
Inspection steps include:

• Removing the valve block and inspecting spool surfaces for scoring.
  
• Testing check valve springs and seats for proper function.
  
• Cleaning or replacing O-rings and seals.
  
• Flushing the block with clean hydraulic fluid before reassembly.
  

A technician in Georgia discovered that a single grain of sand lodged in the check valve seat caused intermittent boom drift. After cleaning and resealing, the issue disappeared.
Hydraulic Fluid and Contamination Control
Hydraulic fluid condition plays a major role in component longevity. Contaminated or degraded fluid accelerates seal wear and valve sticking.
Preventive measures:

• Replace hydraulic fluid every 1,000 hours or annually.
  
• Use OEM-spec filters and change them every 500 hours.
  
• Avoid mixing fluid types—stick to ISO 46 or manufacturer-recommended viscosity.
  
• Store fluid in sealed containers and clean fill ports before servicing.
  

Use a fluid analysis kit to check for water content, metal particles, and viscosity breakdown. High water content can cause seal swelling and reduce lubrication.
Recommendations for Technicians and Owners

• Rebuild lift cylinders every 3,000–4,000 hours or when drift becomes noticeable.
  
• Inspect valve block during major service intervals—especially after fluid contamination.
  
• Keep seal kits and check valve cartridges in stock for quick turnaround.
  
• Train operators to report boom drift early—before it affects safety or productivity.
  
• Document all hydraulic service and component replacements in maintenance logs.
  

Closing Reflections
Boom drift on the Volvo MC135 is a subtle but telling symptom of hydraulic wear. Whether caused by cylinder seal bypass or valve leakage, the solution lies in methodical inspection and proactive service. These machines are built to lift, hold, and maneuver with precision—but only when every seal and spool works in harmony. With clean fluid, tight tolerances, and a watchful eye, the MC135 can continue to perform reliably—keeping its boom steady, its bucket level, and its reputation intact.