Toyota offers two types of power steering on their trucks. There is standard and Progressive Power Steering (PPS). Both types have a recirculating ball system and rotary type hydraulic control valve.
On the PPS, the vehicle speed is detected by a speed sensor and fluid pressure acting on the piston is varied accordingly. When the truck is stopped or when moving at a low speed, fluid pressure is increased to lighten the force required for steering. At high speeds, the pressure is reduced to lessen the amount of assist and provide appropriate steering wheel response.