A landscaping crew reported that a Sany SY65C mini excavator's bucket would slowly curl inward (drift) when lifting heavy loads. The drift was so bad that the operator had to constantly correct the joystick.
The technician diagnosed the "Bucket Control Valve." The bucket circuit uses a "Load-Holding Check Valve" to lock the cylinder position. He removed the main control valve side plate. He found the "Seat" for the load-holding check valve had a small "Crack" across the sealing surface.
The crack was caused by "Water Hammer." The technician found water in the hydraulic oil (the oil looked milky). The water had entered through a loose hydraulic tank filler cap. When the machine operated, the water caused rapid pressure spikes (water is less compressible than oil) which cracked the valve seat.
The repair involved replacing the main control valve housing (as the seat is integral). The hydraulic oil was drained, and the system was flushed with flushing oil to remove water. The filler cap seal was replaced. After refilling with ISO 46 hydraulic oil, the bucket held its position solidly under maximum load.
