Komatsu 850B Master Cylinder Repair Guide

[MikePhua]

The master cylinder in a Komatsu 850B motor grader plays a crucial role in the hydraulic brake system, converting pedal force into hydraulic pressure to ensure safe stopping and control. When issues arise, properly diagnosing and repairing the master cylinder is essential for maintaining machine safety and performance. This detailed guide covers the master cylinder’s repair process, common problems, technical explanations, and maintenance tips to keep the Komatsu 850B operating reliably.
Role and Importance of the Master Cylinder
The master cylinder acts as the heart of the hydraulic brake system on the 850B. When the brake pedal is pressed, the master cylinder pressurizes brake fluid, which transfers force through brake lines to apply brakes on the wheels or components. A failure or leak in the master cylinder reduces braking efficiency and can lead to hazardous situations on the jobsite.
Common Symptoms Indicating Master Cylinder Issues

• Soft or spongy brake pedal feel.
  
• Brake pedal sinking slowly under constant pressure.
  
• Visible hydraulic fluid leaks near the cylinder or pedal area.
  
• Reduced braking force or inconsistent brake function.
  
• The need to pump the brake pedal repeatedly to achieve braking.
  

Recognizing these signs early and addressing cylinder problems can prevent further damage or safety risks.
Master Cylinder Removal Procedure
Due to the master cylinder’s location and connection complexity, removal involves several steps to ensure safety and prevent component damage:

• Remove the left floor plate to access the master cylinder area.
  
• Detach the hydraulic lines or hoses connecting to the master cylinder carefully to avoid fluid spillage.
  
• Remove bolts, lock washers, and nuts attaching the master cylinder to its mounting bracket.
  
• Disconnect the clevis pin and cotter pin linking the push rod to the brake pedal; then remove the master cylinder with its mounting bracket.
  
• In some models, removing the brake pedals and mounting bracket assembly may be necessary for clear access.
  

Inspection and Diagnosis
Once removed, the master cylinder should be inspected for:

• External leaks or cracks in the housing.
  
• Worn or damaged seals, which are the most common sources of internal leaks.
  
• Corrosion or pitting inside the cylinder bore that affects smooth piston movement.
  
• Condition of the push rod clevis and pedal linkage for wear or deformation.
  

Any detected damage, especially to seals or the cylinder bore, usually requires either full rebuild or replacement of the master cylinder.
Repair and Reassembly Steps

• Clean all parts thoroughly to remove dirt and old fluid.
  
• Replace all seals and O-rings with recommended kit components to ensure a secure, leak-free restoration.
  
• Inspect and lubricate the cylinder bore lightly to aid piston movement.
  
• Reassemble the master cylinder pistons carefully, ensuring correct orientation and fit.
  
• Reinstall the master cylinder and mounting bracket to frame, securing all bolts with lock washers.
  
• Reconnect the push rod to the brake pedal using clevis pins and new cotter pins.
  
• Reattach hydraulic lines carefully, then bleed the brake system to remove trapped air, restoring proper hydraulic pressure.
  

Technical Terms Explained

• Master Cylinder: Converts pedal force into hydraulic pressure to operate brakes.
  
• Clevis Pin: A fastener connecting the brake pedal to the push rod on the master cylinder.
  
• Cotter Pin: A locking pin used to secure the clevis pin in place.
  
• Seal Kit: Replacement components including rubber seals and O-rings for restoring hydraulic tightness.
  
• Brake Pedal Free Travel: The distance the pedal moves before resistance is felt, important for proper brake response.
  
• Bleeding the Brake System: Removing air bubbles from hydraulic lines to ensure firm brake activation.
  

Practical Tips and Cautions

• Use only genuine or high-quality seal kits designed for Komatsu 850B to ensure compatibility.
  
• Avoid contaminating hydraulic fluid with dirt or moisture during repair.
  
• Always check the brake pedal free travel adjustment after reassembly to ensure safe operation.
  
• When bleeding brakes, follow manufacturer instructions to maintain system integrity.
  
• Regular inspection of hydraulic lines and master cylinder seals can prevent sudden failures.
  

A User Experience
A maintenance technician shared a story about a Komatsu 850B that developed a slow brake pedal response. Upon inspection, the master cylinder seals were worn out, causing internal leakage. After procuring a genuine rebuild kit, the seals were replaced successfully. Post-repair, the brake pedal felt firm, and the grader’s braking performance was restored, preventing further operational delays. This highlights the importance of routine checks and timely repairs in heavy equipment safety systems.
Summary
Repairing the master cylinder on the Komatsu 850B motor grader is a vital maintenance task that requires careful removal, thorough inspection, seal replacement, and precise reassembly. Understanding the master cylinder’s function and common failure symptoms helps operators maintain brake performance and ensure safety. With proper tools, genuine parts, and diligent procedures, the repair process can extend the life of the master cylinder and keep the grader reliable during demanding work.