Are you having steering problems? Are the pressures to the steer cylinder below specs?

Not sure about this specific truck, but in most trucks, the pressure in the steering circuit is regulated by the pressure valve in the steering unit. The two relief valves on the main control valve regulate the pressure in the hydraulic circuit.
But what is the reason you want to increase the pressure? Is it because of a gradual loss of pressure? In that case, the sollution to your problem would probably to replace the steering unit