heavy steering

[yangyang]

Hello, I have a problem about heavy steering.
My car is 96 Ford Traurus. Yesterday, I filled a little bit power steering fluid because it is low and feel heavy sometimes. but after filling, things do not change. it is still and even more heavy and frequent than before. but if shut off the car and restart again, the steering is fine for a while and then it becomes heavy again. Now i doulbt maybe areation is involved. but i dont know. could anybody help me? Thanks