Hi Gareth, I'm thinking with Eaton on the counterweight it may be a GDP40AB type have a look on the top of the frame were the counterweight is fastend to the unit left side or right side and you may find the serial no stamped into the frame. I would check king pin thrust bearings and it may have needle roller bearings that support the king pins check them the caps can split of them and you can loose the needle rollers. Maybe you could T a pressure gauge into the steering cylinder at the steer cylinder you should be looking for 1500lbs at least have a listen to see if you can here oil being pushed past relief valve. It could be the steering orbit roll has past its sell by date and is partially seized I've had them go like that. If the steer cylinder pickings had worn out the steering would be lighter and steering would be slower to operate.

Regards Nero.