Brake Pedal Loses Pressure Without Proper Bleeding and Adjustment in Hydraulic Drum Brake Systems

[MikePhua]

Understanding the Brake System in Older Hydraulic Equipment
Many older cranes and construction machines, such as the Grove RT-58C rough terrain crane, use a hydraulic drum brake system. These systems rely on a master cylinder to generate hydraulic pressure, which is transmitted through brake lines to wheel cylinders that actuate brake shoes against a drum. Over time, air can enter the system during maintenance or component replacement, leading to a spongy pedal or complete loss of braking force unless the pedal is pumped repeatedly.
In the case of a full brake system overhaul—including new linings, wheel cylinders, lines, and a master cylinder—air entrapment is almost inevitable. Even with fluid present at the bleeder valves, residual air pockets can cause the pedal to feel soft or require pumping to build pressure.
Why the Pedal Goes Soft After Sitting
A common symptom is a firm pedal immediately after bleeding, which then goes soft after a few minutes of rest. This typically indicates that the brake shoes are not adjusted close enough to the drum surface. When the shoes are too far from the drum, the wheel cylinder pistons must travel farther to make contact, consuming more fluid volume and reducing pressure at the pedal.
This issue is compounded if the master cylinder has a small bore or if the system lacks residual pressure valves. These valves are designed to maintain slight pressure in the lines to keep the wheel cylinder cups expanded and reduce air ingress.
Proper Bleeding Procedure for Hydraulic Drum Brakes
To ensure a firm pedal, follow this sequence:

• Begin with the wheel farthest from the master cylinder (usually right rear)
  
• Use a clear hose submerged in brake fluid to monitor air bubbles
  
• Have an assistant pump the pedal slowly and hold it down
  
• Open the bleeder valve, allow fluid and air to escape, then close the valve before releasing the pedal
  
• Repeat until no air bubbles are visible
  
• Move to the next wheel and repeat the process
  

If air continues to appear, inspect all fittings and connections for leaks. Even a small seep can allow air to re-enter the system.
Adjusting Brake Shoes for Optimal Performance
After bleeding, manually adjust the brake shoes using the star wheel adjuster:

• Turn the adjuster until the shoes lightly drag against the drum
  
• Back off slightly to prevent overheating during operation
  
• Repeat for all wheels
  

This step is critical. Without proper adjustment, the system will always feel underpowered, regardless of how well it’s bled.
A Real-World Example from the Field
An operations manager in Texas reported that after replacing every brake component on a Grove RT-58C, the pedal still required pumping to build pressure. Fluid sprayed from the bleeders, but the pedal would sink after a few minutes. The issue was resolved not by re-bleeding, but by adjusting the brake shoes closer to the drums. Once adjusted, the pedal remained firm and responsive.
Additional Tips for Long-Term Brake Health

• Use DOT-rated brake fluid compatible with your system
  
• Replace rubber hoses if they show signs of swelling or cracking
  
• Periodically check for leaks at wheel cylinders and master cylinder
  
• Store equipment in dry conditions to prevent moisture contamination
  

Conclusion
A soft or fading brake pedal after a full rebuild is often not due to air alone but to improper shoe adjustment. In hydraulic drum brake systems, mechanical setup is just as important as fluid dynamics. By combining thorough bleeding with precise shoe adjustment, operators can restore full braking performance and ensure safe operation in demanding environments.