Start the engine, rotate the steering wheel several times to raise the temperature of the working fluid in the system to 50-60°C.
With the engine running, turn the steering wheel fully left and right several times.
Check for foaming or a milky tint to the hydraulic fluid in the power steering reservoir. Check the difference in fluid levels with the engine running and not running. If this difference is more than 5 mm, it is necessary to bleed air from the power steering system.