Racetrack Junkies

i've changed my power steering fluid
the prob is the day i've change my power steering fluid, my steering runs smooth i the steering is very light
but after 1 week, my steering return to the previous condition that the steering is hard
what is the prob?hw can i do with it
i've asked my friend and he says got air inside the power steering hose and he teach me to repair it. he teach me to turn my steering over and over again when the engine is on. i've tried but nthng happens