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Introduction to Hydraulic Air Entrapment
Air trapped inside a hydraulic system is one of the most common causes of hesitation, jerky movement, and inconsistent cylinder response in older excavators. When a machine undergoes major hydraulic work—such as pump rebuilding, hose replacement, or valve block servicing—it is normal for air to enter the system. However, if the air does not purge correctly, the machine may exhibit symptoms that mimic mechanical failure.
Hydraulic systems rely on incompressible fluid to transmit force. Air, by contrast, is highly compressible. Even a small amount of trapped air can create a momentary loss of pressure, resulting in a “dead spot” where a cylinder stops moving before pressure builds again.
Terminology Note
Air entrapment: Air bubbles trapped within hydraulic lines or components.
Bleeder: A valve used to release trapped air from a hydraulic circuit.
Regeneration circuit: A hydraulic design that redirects return oil to assist cylinder movement, improving speed and reducing dead spots.
Stick: The upper boom section of an excavator, also called the dipper arm.
Symptoms Observed After Hydraulic Pump Rebuild
A typical example involves an older excavator—such as a Cat 225LC—where the hydraulic pumps were rebuilt and several hoses replaced. After reassembly, the machine exhibited a hesitation during bucket curl and stick retraction. The outward movements worked smoothly, but inward movements paused for several seconds before resuming.
This hesitation is a classic sign of air trapped in one side of the cylinder circuit. Because inward and outward movements use different chambers and different flow paths, it is possible for one direction to purge successfully while the other retains air.
In this case, bleeding the outward circuit improved performance, but the inward circuit continued to show a dead spot.
Why Bleeding Procedures Differ Between Machines
Different excavator manufacturers use different hydraulic architectures. For example, Komatsu machines such as the PC200‑6 require a specific bleed sequence to purge air efficiently. If the sequence is not followed, air can remain trapped in high points of the circuit, causing prolonged hesitation.
Older Caterpillar machines, including the 225 and 235 series, do not always have regeneration circuits found in newer models. Without regeneration, the return oil does not assist the cylinder during certain movements, making any trapped air more noticeable.
This explains why some machines exhibit a pause at the bottom of the stick’s arc of travel—especially when pulling the stick inward. The pump must catch up and build pressure before movement resumes.
Why Air Does Not Always Purge Automatically
Many operators assume that air will eventually work its way back to the hydraulic tank. While this is partially true, several factors can prevent complete purging:
Regeneration Circuit Influence on Dead Spots
Some excavators use regeneration circuits to improve stick speed and reduce cavitation. These circuits redirect return oil back into the cylinder instead of sending it to the tank. Machines without regeneration may experience a momentary loss of pressure when the cylinder transitions between flow paths.
A mechanic noted that older Cat 235 excavators often pause at the bottom of the stick’s travel arc because the pump needs a moment to catch up. This behavior is normal and becomes more noticeable when air is present.
Potential Misdiagnosis and Real-World Experiences
A farmer in Alberta once described a similar issue on his excavator. After replacing several hoses, the stick would hesitate during retraction. He assumed it was air, but the real cause was insufficient return oil flow due to a partially blocked regeneration valve. Once cleaned, the hesitation disappeared.
Another operator in Tennessee reported that his machine worked fine for years until a pump rebuild introduced air into the system. Despite hours of operation, the air did not purge completely until the correct bleeder was identified.
These stories highlight a common theme: hydraulic systems are sensitive to both air and flow restrictions, and symptoms can overlap.
Best Practices for Removing Air from Excavator Hydraulics
To ensure proper purging, technicians recommend:
Company Background and Equipment History
Caterpillar’s 200‑series excavators, including the 225LC and 235, were produced during the late 1970s and 1980s. These machines helped Caterpillar expand its global excavator market share, especially in North America and Asia. The 225LC became known for its robust hydraulic pumps and simple valve architecture, which made it popular among contractors and rental fleets.
By the mid‑1980s, Caterpillar was producing thousands of excavators annually, and the 200‑series contributed significantly to the company’s growth. Although newer models introduced advanced regeneration circuits and electronic controls, the older machines remain widely used due to their mechanical simplicity and ease of repair.
Conclusion
Air in a hydraulic system can cause hesitation, dead spots, and inconsistent cylinder movement, especially after major repairs. While bleeding procedures can resolve most issues, older excavators may require specific sequences or additional troubleshooting due to their hydraulic architecture. Understanding the role of regeneration circuits, valve block design, and proper bleeding techniques is essential for restoring smooth operation.
With careful attention and methodical purging, even older machines like the Cat 225LC can return to reliable performance. These excavators continue to serve operators worldwide, a testament to their durable design and Caterpillar’s long-standing engineering legacy.
Air trapped inside a hydraulic system is one of the most common causes of hesitation, jerky movement, and inconsistent cylinder response in older excavators. When a machine undergoes major hydraulic work—such as pump rebuilding, hose replacement, or valve block servicing—it is normal for air to enter the system. However, if the air does not purge correctly, the machine may exhibit symptoms that mimic mechanical failure.
Hydraulic systems rely on incompressible fluid to transmit force. Air, by contrast, is highly compressible. Even a small amount of trapped air can create a momentary loss of pressure, resulting in a “dead spot” where a cylinder stops moving before pressure builds again.
Terminology Note
Air entrapment: Air bubbles trapped within hydraulic lines or components.
Bleeder: A valve used to release trapped air from a hydraulic circuit.
Regeneration circuit: A hydraulic design that redirects return oil to assist cylinder movement, improving speed and reducing dead spots.
Stick: The upper boom section of an excavator, also called the dipper arm.
Symptoms Observed After Hydraulic Pump Rebuild
A typical example involves an older excavator—such as a Cat 225LC—where the hydraulic pumps were rebuilt and several hoses replaced. After reassembly, the machine exhibited a hesitation during bucket curl and stick retraction. The outward movements worked smoothly, but inward movements paused for several seconds before resuming.
This hesitation is a classic sign of air trapped in one side of the cylinder circuit. Because inward and outward movements use different chambers and different flow paths, it is possible for one direction to purge successfully while the other retains air.
In this case, bleeding the outward circuit improved performance, but the inward circuit continued to show a dead spot.
Why Bleeding Procedures Differ Between Machines
Different excavator manufacturers use different hydraulic architectures. For example, Komatsu machines such as the PC200‑6 require a specific bleed sequence to purge air efficiently. If the sequence is not followed, air can remain trapped in high points of the circuit, causing prolonged hesitation.
Older Caterpillar machines, including the 225 and 235 series, do not always have regeneration circuits found in newer models. Without regeneration, the return oil does not assist the cylinder during certain movements, making any trapped air more noticeable.
This explains why some machines exhibit a pause at the bottom of the stick’s arc of travel—especially when pulling the stick inward. The pump must catch up and build pressure before movement resumes.
Why Air Does Not Always Purge Automatically
Many operators assume that air will eventually work its way back to the hydraulic tank. While this is partially true, several factors can prevent complete purging:
- Air pockets trapped in high points of the valve block
- Air trapped behind cylinder pistons
- Incorrect bleeding sequence
- Low flow conditions that fail to push air through the system
- Manuals that combine multiple models, causing confusion about correct bleeder locations
Regeneration Circuit Influence on Dead Spots
Some excavators use regeneration circuits to improve stick speed and reduce cavitation. These circuits redirect return oil back into the cylinder instead of sending it to the tank. Machines without regeneration may experience a momentary loss of pressure when the cylinder transitions between flow paths.
A mechanic noted that older Cat 235 excavators often pause at the bottom of the stick’s travel arc because the pump needs a moment to catch up. This behavior is normal and becomes more noticeable when air is present.
Potential Misdiagnosis and Real-World Experiences
A farmer in Alberta once described a similar issue on his excavator. After replacing several hoses, the stick would hesitate during retraction. He assumed it was air, but the real cause was insufficient return oil flow due to a partially blocked regeneration valve. Once cleaned, the hesitation disappeared.
Another operator in Tennessee reported that his machine worked fine for years until a pump rebuild introduced air into the system. Despite hours of operation, the air did not purge completely until the correct bleeder was identified.
These stories highlight a common theme: hydraulic systems are sensitive to both air and flow restrictions, and symptoms can overlap.
Best Practices for Removing Air from Excavator Hydraulics
To ensure proper purging, technicians recommend:
- Following the manufacturer’s exact bleeding sequence
- Bleeding both sides of each cylinder circuit
- Cycling each function slowly to avoid cavitation
- Keeping the hydraulic tank full to minimize air ingestion
- Checking for manuals that cover multiple models and verifying which diagrams apply
- Inspecting hoses and fittings for micro-leaks that can draw air under suction
Company Background and Equipment History
Caterpillar’s 200‑series excavators, including the 225LC and 235, were produced during the late 1970s and 1980s. These machines helped Caterpillar expand its global excavator market share, especially in North America and Asia. The 225LC became known for its robust hydraulic pumps and simple valve architecture, which made it popular among contractors and rental fleets.
By the mid‑1980s, Caterpillar was producing thousands of excavators annually, and the 200‑series contributed significantly to the company’s growth. Although newer models introduced advanced regeneration circuits and electronic controls, the older machines remain widely used due to their mechanical simplicity and ease of repair.
Conclusion
Air in a hydraulic system can cause hesitation, dead spots, and inconsistent cylinder movement, especially after major repairs. While bleeding procedures can resolve most issues, older excavators may require specific sequences or additional troubleshooting due to their hydraulic architecture. Understanding the role of regeneration circuits, valve block design, and proper bleeding techniques is essential for restoring smooth operation.
With careful attention and methodical purging, even older machines like the Cat 225LC can return to reliable performance. These excavators continue to serve operators worldwide, a testament to their durable design and Caterpillar’s long-standing engineering legacy.
