There is a signal line to the counterbalance valve from the block on the centre of the torque tube that contains an orificed plug ref #31 right next to the elbow coming out of the block. It's shown as Ref #66 on the Hydraulic Schematic. Has the orifice become blocked or maybe even the orifice plug been left out completely.? Also check out items ref #65 & 68 on the hydraulic schematic, especially #68 which is a vent from the rod end of the hoist cylinders.