A construction site reported that a Caterpillar 320D2 excavator would swing freely (coast) when the machine was parked on a slope. The "Swing Parking Brake" (located on the swing motor) failed to hold the upper structure.
The technician tested the brake release pressure. The swing motor uses a "Hydraulically Released, Spring Applied" brake. To release the brake, pilot pressure (approx. 30 bar) pushes a piston against a spring. He connected a gauge to the brake port. The pressure built up to 30 bar, but the brake didn't release. When he released the joystick, the pressure dropped to 0, but the machine still swung (brake didn't engage).
He disassembled the swing motor. He found the "Brake Piston O-Ring" was cut. The cut allowed hydraulic oil to leak into the brake housing chamber. Because the housing was full of oil, the "Viscous Drag" of the oil slowed the machine, but the mechanical brake discs weren't clamping.
Additionally, he found the "Friction Discs" were glazed. The operator had a habit of "riding the swing brake" (holding the brake while swinging) to control speed. This constant slipping had polished the friction surfaces, reducing the holding torque.
The repair involved replacing the brake piston seal and the friction discs. The hydraulic tank was drained and flushed because the debris from the cut seal had circulated. He also adjusted the "Swing Speed Sensor" to remind the operator to use the "Swing Pedal" for speed control, not the brake.
