Clark C40B Forklift Brake Troubleshooting and Restoration

[MikePhua]

Clark’s Industrial Legacy and the C40B Forklift
Clark Material Handling Company, founded in 1917, pioneered the modern forklift with its Tructractor. By the 1960s, Clark had become a global leader in industrial lift trucks, producing over 500,000 units by the end of that decade. The Clark C40B, introduced in the mid-1960s, was a rugged internal combustion counterbalance forklift designed for warehouse and yard use. With a 4000 lb lifting capacity and a compact chassis, it became a staple in manufacturing plants and distribution centers across North America.
The C40B featured mechanical drum brakes, a hydraulic master cylinder, and an inching valve system that allowed precise control during load placement. While robust, its brake system requires careful attention during restoration, especially when mixing old components with modern replacements.
Symptoms of Brake Failure and Initial Repairs
A common issue with aging C40B forklifts is a soft or unresponsive brake pedal. In one restoration case, the owner replaced the master cylinder, wheel cylinders, brake shoes, hydraulic lines, and the inching valve cup seal. Despite bleeding the system thoroughly, the pedal still traveled to the floor before engaging weak braking force.
Terminology note:

• Master Cylinder: Converts pedal force into hydraulic pressure to actuate brakes.
  
• Wheel Cylinder: Located at each wheel, it receives hydraulic pressure and pushes brake shoes against the drum.
  
• Inching Valve: Allows partial disengagement of the transmission for precise maneuvering without full braking.
  

This kind of long pedal travel and vague braking response typically points to either air in the system, incorrect pushrod length, or mismatched components.
Pushrod Length and Master Cylinder Compatibility
One overlooked factor in brake restoration is the pushrod that connects the pedal linkage to the master cylinder. If the original pushrod is reused with a modern aftermarket master cylinder, it may be too short or too long.
Consequences of incorrect pushrod length:

• Too short: The piston doesn’t travel far enough to build pressure.
  
• Too long: The piston remains partially depressed, preventing fluid from returning and causing internal vacuum or blockage.
  

Solution:

• Measure the original pushrod and compare it to the one supplied with the new master cylinder.
  
• Ensure the piston fully retracts when the pedal is released.
  
• Adjust or fabricate a new pushrod if necessary, using hardened steel and thread-locking compound.
  

In one case from Mississippi, a retired mechanic discovered that the aftermarket master cylinder had a deeper piston bore. By switching to the pushrod included with the new cylinder and adjusting the pedal linkage, the brakes immediately firmed up.
Bleeding the Brake System and Fluid Selection
Bleeding drum brake systems on vintage forklifts can be tricky due to hidden air pockets and long hydraulic lines. The C40B uses a single-line system with branch circuits to each wheel cylinder.
Recommended bleeding procedure:

• Start with the wheel furthest from the master cylinder.
  
• Use a vacuum bleeder or gravity method if pressure bleeding fails.
  
• Tap the lines gently with a rubber mallet to dislodge trapped air.
  
• Repeat the cycle twice to ensure full evacuation.
  

Fluid choice matters. While DOT 3 brake fluid is standard, older seals may degrade with modern additives. If the system was rebuilt with new seals, DOT 3 or DOT 4 is acceptable. Always avoid mixing fluid types.
Inching Valve Interaction and Brake Feel
The inching valve on the C40B is hydraulically linked to the brake pedal. When the pedal is pressed, the valve partially disengages the transmission to allow slow movement. If the valve cup seal is worn or misaligned, it can interfere with brake pressure buildup.
Signs of inching valve interference:

• Brake pedal activates transmission disengagement before building pressure.
  
• Pedal feels spongy or inconsistent.
  
• Brakes engage only at full pedal depression.
  

Solution:

• Inspect the inching valve cup seal for wear or improper seating.
  
• Replace with OEM-grade seals or matched equivalents.
  
• Ensure the valve linkage is synchronized with the brake pedal stroke.
  

A Story from the Warehouse Floor
In Massachusetts, a vintage Clark C40B was used to move steel stock in a fabrication shop. After a full brake overhaul, the operator still found the pedal unreliable. A local technician traced the issue to a mismatched pushrod and a misaligned inching valve. After fabricating a new rod and reseating the valve seal, the forklift regained full braking power. It went on to serve another decade before being retired in favor of a newer Clark S-Series.
Additional Checks and Long-Term Maintenance
To ensure reliable braking on a restored C40B, consider these steps:

• Inspect brake drums for scoring or oval wear. Resurface or replace if needed.
  
• Check pedal bushings and return springs for wear.
  
• Replace flexible hydraulic lines with braided steel if operating in high-heat environments.
  
• Test brakes under load to confirm even engagement on both sides.
  

Preventive maintenance tips:

• Flush brake fluid annually to prevent moisture buildup.
  
• Grease pedal linkage and inching valve pivot points.
  
• Monitor wheel cylinder seals for seepage.
  

Conclusion
Restoring the brake system on a Clark C40B forklift requires more than just replacing parts—it demands attention to compatibility, linkage geometry, and hydraulic behavior. With careful diagnosis and a few custom adjustments, even a 60-year-old machine can regain safe and responsive braking. These forklifts were built to last, and with the right care, they continue to earn their keep in shops and yards across the country.