these old units have a 'priority valve' in the system that diverts some of the hydraulic fluid to the steering, usually when the steering acts up and it affects the hydraulic functions at the same time this valve is generally sticking and should be cleaned out (or changed if damaged).
Need to know the model and serial number of the machine to be certain where the valve is located. Most of the older trucks it was located underneath the floor plate, but at a point the newer models moved that into the hydraulic control valve. The very old trucks the steering was solely controlled independently from the hydraulic system by the orbital steering valve but from your description this doesn't sound like your truck is that old.