A quarry operator reported that a JCB 385D excavator would slowly creep forward when parked on a flat surface with the travel levers in neutral. Additionally, when lifting a heavy load, the boom would drop by about 10 cm immediately after the lever was released.
The technician initially suspected internal leakage in the boom cylinder, but a drift test with the boom raised and the engine off showed the cylinder holding firmly. He then focused on the "Central Swivel Joint" (rotary manifold), which routes hydraulic oil from the stationary upper structure to the traveling lower chassis.
He disconnected the travel motor return lines and found a steady flow of oil even with the travel levers in neutral. He disassembled the central swivel joint and found the internal "O-Ring Seals" and "Backup Rings" separating the boom raise circuit from the travel circuits were completely flattened and cut. High-pressure oil from the boom circuit was bleeding across the worn grooves directly into the travel motor lines. This cross-leak pressurized the travel motor lines unintentionally, causing the tracks to creep. The boom drop occurred because the oil meant to hold the boom was escaping into the travel circuit, causing a momentary pressure decay until the main relief valve compensated.
The wear was traced to a lack of chassis grease. The swivel joint's external grease cavity had dried out, allowing dirt to enter the bearing, which scored the internal barrel. The repair involved replacing the complete central swivel joint seal kit and polishing the internal barrel with fine emery cloth to remove the scoring. The external grease fitting was pumped until fresh grease expelled the old, dirty grease. After reassembly, the excavator parked firmly without creep, and the boom held its load perfectly.
