I have an older Towmotor 30,000 lbs forklift.

It does have power steering where a hydraulic cylinder provides
assistance. It has a power steering gearbox of unknown (to me)
construction.

The problem that it has developed is that power steering works great
when I turn steering wheel to the left, and it does not work when I
attempt to turn it right. Instead, turning right requires an enormous
effort.

What could the problem be here, thanks. Maybe there are some valves in
the power steering gearbox that malfunction?

i