Hello all, hoping someone might be able to help with a Hyster forklift issue:

We have a Hyster forklift fitted with a Perkins 203 direct injection engine, cannot find serial number its about 25 yrs old, ETN on the back weight and Yale on the side covers.

Problem is that the steering has become very heavy, this happened very suddenly and the other hydraulics work ok. Wondering if anyone had any suggestions and it would be very helpful if someone had a diagram of the steering system.