1982 Toyota Landcruiser Steering Problem

The power steering pump provides power to the gear box. If the pump is providing power as it should and no valves are getting stuck the gear box may be bad

One thing I can think of that would test to see if it is a valve issue or the gear box itself is remove the hydralic lines from the gear box. You will need to figure out a way to get the fluid that is pumped out to the gear box back to the resivior. If when you turn the wheel it is still harder to turn the wheel one way or the other then it is the valves and not the gear box. If it is easy to turn both ways then you will need a gear box.

I'm not familiar with your particular vehicle, but if the valves are in the gear box then the box will have to be rebuilt to change them. I have never heard of a method of getting the valves cleaned up by themselves.