A power steering pressure line carries high-pressure oil from the power steering pump to the steering gear. A second, low-pressure steering line then carries the depressurized oil from the steering gear back to the pump or the pump's oil reservoir, where it is pressurized and used again.
A car's power steering pump is a type of rotary vane pump that pressurizes oil using spinning blades. When the blades spin, they force a large volume of power steering oil to occupy a small area, which increases the oil's pressure. As the car travels faster, the rate at which the pump's blades spin increases, which increases the pressure in the line. Power steering pressure lines are made of durable polymers and held in place with compression fittings to cope with these pressures.
The pressure line carries oil to the steering gear, which does most of the work in a power steering system. When the driver turns the steering wheel, she also turns the torsion bar inside the steering gear. The torsion bar attaches to the pinion gear, which turns the wheels. As the driver turns the steering wheel farther in either direction, the torsion bar twists more, increasing the amount of oil that the pressure line carries into the steering gear. The pressure of the oil inside the steering gear helps turn the pinion gear so that turning the wheel requires minimal physical effort from the driver.