A heavy construction crew reported that a Kobelco SK350LC-8 excavator was experiencing a dangerous combination of symptoms. First, when the boom was raised to full height and the engine was shut off, the boom would crash down to the ground within 30 seconds. Second, the pilot control system felt "spongy," and the machine would sometimes creep forward even when the travel levers were in neutral.
The technician approached this as two separate hydraulic issues. He first tested the main hydraulic pumps, which were performing to spec. To diagnose the boom drop, he performed a "cylinder bypass test" by pressurizing the boom cylinder and blocking the return line. The cylinder held pressure, ruling out internal piston seal leakage. This pointed to the main control valve. He removed the main control valve cover and inspected the "boom load check valve." The load check valve is designed to hold the hydraulic column of oil in the cylinder when the spool is centered. Upon disassembly, he found the spring behind the check valve poppet was broken into three pieces. This prevented the poppet from seating, allowing oil to bleed back to the tank, causing the drift.
The second issue-the spongy pilot system-required a different approach. The pilot pump supplies low-pressure oil (approx. 3-4 MPa) to the joystick controllers. The technician located the pilot isolation valve, which is activated by a solenoid to enable pilot pressure only when the safety lever is raised. He found a weeping oil leak from the valve housing. Upon closer inspection with a magnifying glass, he discovered a hairline crack running through the casting of the valve body, extending into the internal pilot passage.
The crack was caused by a previous impact on the side of the machine's chassis. When the hydraulic oil heated up, the crack expanded, allowing air to be drawn into the pilot system (cavitation). This air caused the "spongy" feel and the drifting travel motors, as the air compresses instead of pushing the spool valves firmly into neutral. The repair involved replacing the cracked pilot isolation valve housing and replacing the broken load check spring in the main control valve. The hydraulic system was then purged of air, and the boom drop and pilot sponginess were fully resolved.
