Technician did checking by following Found Arm hadlowering rate greater than 2 mm/min then suspect failure cause is Hitch valve (SJ19722). After replaced a brand new Hitch valve anddid testing again found Arm had lowering rate 0.2 mm/min then suspect the Cylinder/Piston. After removed Cylinder found Piston wearring damaged then replace a brand new Back-up Ring (R232175) a Wear Ring (R225912).