A contractor reported that a Komatsu PC300LC-8 excavator had a delay when starting to swing. The operator would move the joystick, and the machine would pause for 2-3 seconds before swinging, often with a "jerk." The swing speed was normal once it started moving.
The PC300LC-8 uses a hydraulically released parking brake for the swing motor. When the swing lever is moved, a pilot signal sends oil to the brake piston to release it. The technician tested the swing brake solenoid valve located on the main control valve. The valve was clicking, but the pilot pressure gauge showed a delay in pressure build-up.
The solenoid valve was removed and disassembled. The spool was stuck with a sticky residue, likely from degraded hydraulic oil. The spool was not moving freely to redirect pilot oil to the brake release port, causing the delay. The "jerk" was the sudden engagement of the swing motor while the brake was still partially applied.
Furthermore, the hydraulic tank breather cap was checked and found to be completely blocked with mud. This was creating a slight vacuum in the tank, which was restricting the flow of oil to the swing motor pump, contributing to the cavitation and jerky motion. The repair involved cleaning the swing brake solenoid spool with solvent and replacing the hydraulic tank breather. The swing function became instant and smooth after the repair.
