Diagnosing Cylinder Drift in Vintage Michigan 175-A Loaders

[MikePhua]

The Michigan 175-A and Its Historical Significance
The Michigan 175-A wheel loader was produced during the 1960s by Clark Equipment Company, a manufacturer that played a pivotal role in shaping mid-century earthmoving machinery. Known for their robust drivetrains and mechanical simplicity, Michigan loaders were widely adopted by municipalities, logging outfits, and construction firms across North America. The 175-A featured a torque converter transmission, articulated steering, and a hydraulic system capable of powering large-capacity buckets and lifting arms.
Though production ceased decades ago, many units remain operational due to their overbuilt components and ease of repair. However, as these machines age, hydraulic issues such as cylinder drift become increasingly common, especially in systems with original valve bodies and worn spools.
What Causes Cylinder Drift
Cylinder drift refers to the gradual, unintended movement of a hydraulic cylinder when the control lever is in the neutral position. In the case of the Michigan 175-A, the boom slowly lowers even while the engine is running, and the bucket flattens overnight when parked. This behavior points to internal leakage within the hydraulic control valve or the cylinders themselves.
Terminology annotation:

• Cylinder Drift: Unintended movement of a hydraulic actuator due to internal leakage or valve failure.
  
• Valve Spool: A precision-machined component that slides within a valve body to direct hydraulic flow.
  
• Port Relief Valve: A pressure-limiting device that protects individual hydraulic circuits from overload.
  
• Load Check Valve: A one-way valve that prevents backflow and maintains cylinder position under load.
  

Evaluating the Severity and Repair Options
Operators must first assess whether the drift poses a safety or operational risk. In this case, the loader still lifts and curls effectively, suggesting that the pump and main relief valves are functioning. However, the drift forces the operator to work quickly when lifting suspended loads, which can be hazardous.
The most likely cause is wear between the valve spool and the valve body bore. Over decades of use, the metal-to-metal clearance increases, allowing pressurized fluid to bypass the spool and leak into the return circuit. Unlike cylinder seals, these tolerances cannot be restored without replacing the entire valve assembly.
Testing for Internal Leakage
To isolate the source of drift, technicians can perform a cylinder isolation test:

• Install ball valves at the base ports of the boom cylinders
  
• Raise the boom and shut off the valves
  
• Observe the boom position after several hours
  

If the boom remains elevated, the cylinders are holding pressure and the leak is in the valve. If the boom drops, the cylinder seals are compromised. This test helps determine whether valve replacement alone will resolve the issue.
Understanding Relief Valve Behavior
Some confusion arises around the role of port relief valves. If a relief valve is leaking, it typically does so slowly and only when the system is idle. It does not prevent the loader from lifting heavy loads, as the main relief valve governs maximum system pressure. A faulty port relief may contribute to drift but is unlikely to be the sole cause.
In vintage systems like the 175-A, relief valves may be integrated into the valve body and not sold separately. However, they can sometimes be removed and inspected. A broken spring or damaged seat can cause leakage, and replacement springs or poppets may be sourced from hydraulic suppliers if dimensions are known.
Valve Spool Limitations and Rebuild Challenges
Unlike hydraulic cylinders, valve spools do not use packings or seals. They rely on a precision fit within the valve bore. Once this fit is compromised, internal leakage becomes inevitable. Rebuilding a valve spool would require re-machining the bore or fabricating an oversized spool—neither of which is practical without specialized equipment.
Some operators have successfully retrofitted modern valves onto vintage machines. This requires:

• Matching flow rates and pressure ratings
  
• Adapting mounting brackets and linkage geometry
  
• Re-routing hydraulic lines to match new port configurations
  

While not original, this approach can restore functionality and reduce drift, especially when OEM parts are no longer available.
Inspecting Load Check Valves
In some Michigan valve designs, a check valve plunger is integrated into the spool assembly. This component seals against a beveled seat and prevents backflow when the control lever is in neutral. If the check valve is damaged or the spring is broken, drift may occur even if the spool is intact.
Recommendations for inspection:

• Remove the check valve and inspect the sealing surface for pitting or wear
  
• Test spring tension and replace if weak or broken
  
• Clean all components thoroughly before reassembly
  

Unlike the spool, the check valve may be serviceable with basic tools and replacement parts.
Conclusion
Cylinder drift in the Michigan 175-A is a manageable issue rooted in hydraulic valve wear and aging components. While valve spool clearance cannot be repaired, targeted testing and inspection of relief and check valves may offer partial solutions. For operators committed to preserving these classic machines, retrofitting modern valves or sourcing used assemblies remains a viable path. The key is understanding the hydraulic architecture and approaching repairs with methodical precision—honoring the legacy of a machine built to endure.