without a model number this may prove difficult to even guess at

first determine if this has a pump mounted on the engine for power steering or if it uses the hydraulic system

an easy way to determine this is to trace the lines from the steering orbital valve
there should be 2 lines to the cylinder, 1 line from the pressure source and 1 drain back line

some older units had a relief valve on the orbital valve, if this one has that then pull it and check for broken springs.
other units utilized a priority valve mounted between the control valve auxillary pressure outlet and orbital valve where it got it's pressure from, they are known to stick and cause steering failure.

and if its a really old design the steering cylinder is mounted in the frame along side the engine with a drag link and mechanical steer axle, those cylinder assys had a valve on those that controlled steering direction and if they failed the steering may go out.

without a model this is just a best guess at what to look for