Bleeding the Brake System on a Case 621D Loader

[MikePhua]

The Case 621D Loader Platform
The Case 621D wheel loader was introduced in the early 2000s as part of Case Construction Equipment’s D-series lineup, designed for mid-size material handling in construction, quarrying, and municipal operations. With an operating weight of approximately 12,000 kg and a bucket capacity ranging from 2.4 to 3.1 cubic meters, the 621D offered a balance of power, maneuverability, and service accessibility.
Powered by a Case 6.7L turbocharged diesel engine delivering around 150 horsepower, the 621D featured a four-speed powershift transmission, Z-bar linkage for breakout force, and a sealed wet disc brake system. The wet disc brakes, immersed in hydraulic fluid, provide consistent stopping power and long service life—but they require careful bleeding when air enters the system.
Understanding the Brake System Architecture
The 621D uses a closed-loop hydraulic braking system with the following components:

• Dual brake master cylinders
  
• Brake accumulators charged with nitrogen
  
• Wet disc brakes integrated into the axle hubs
  
• Hydraulic pump and priority valve
  
• Brake pedal valves and return springs
  
• Bleed ports located on each axle housing
  

The system relies on hydraulic pressure stored in the accumulators to actuate the brakes. Air in the system can cause spongy pedal feel, delayed response, or complete brake failure. Bleeding restores full hydraulic integrity by removing trapped air.
Symptoms of Air in the Brake System
Operators may notice:

• Soft or sinking brake pedal
  
• Delayed stopping response
  
• Brake warning light activation
  
• Audible hissing or gurgling during pedal application
  
• Reduced braking force on one side
  

In one case, a quarry operator in Alberta reported that his 621D loader rolled slightly when parked on a slope. Inspection revealed air in the rear axle brake circuit, introduced during a recent hydraulic hose replacement.
Bleeding Procedure and Safety Precautions
Before bleeding:

• Park the machine on level ground
  
• Engage the parking brake
  
• Block wheels securely
  
• Shut down engine and relieve hydraulic pressure
  
• Wear eye protection and gloves
  

Bleeding steps:

1. Locate the bleed ports on each axle housing—typically near the brake caliper area
   
2. Attach clear tubing to the bleed screw and route to a container
   
3. Open the bleed screw slightly
   
4. Have a second person press the brake pedal slowly
   
5. Observe fluid flow—look for bubbles or foam
   
6. Continue until clear fluid flows without air
   
7. Tighten bleed screw while pedal is depressed
   
8. Repeat for all wheels, starting with the furthest from the master cylinder
   

If the loader has dual accumulators, ensure both are charged to the correct nitrogen pressure (typically 1,500–2,000 psi). Use a certified charging kit and follow manufacturer specs.
Common Mistakes and Field Corrections
Avoid these errors:

• Bleeding with engine running—can cause pressure surges
  
• Over-tightening bleed screws—may crack housing
  
• Using contaminated fluid—introduces new air or debris
  
• Ignoring accumulator charge—leads to false bleeding results
  
• Failing to cycle the pedal between bleeds—traps air in valves
  

A technician in Texas once replaced a brake hose but forgot to recharge the accumulator. Despite bleeding the system, the brakes remained soft. After charging the accumulator to spec, the brakes returned to full strength.
Preventive Maintenance and Recommendations
To maintain brake performance:

• Inspect fluid level weekly
  
• Replace brake fluid every 1,000 hours or annually
  
• Test accumulator pressure quarterly
  
• Check pedal travel and spring tension monthly
  
• Clean bleed ports before opening
  
• Use Case-approved hydraulic brake fluid
  

Installing a pressure gauge on the accumulator circuit can help monitor system health. Some fleets retrofit warning buzzers for low accumulator pressure to prevent unnoticed failures.
Conclusion
Bleeding the brake system on a Case 621D loader is a critical maintenance task that ensures safe operation and responsive stopping power. With a methodical approach, proper tools, and attention to accumulator pressure, operators can restore full braking function and prevent costly downtime. In the demanding world of material handling, reliable brakes are not optional—they’re the foundation of control, safety, and confidence.