Leak in power steering mechanism

A few years ago I had the power steering pump and both the high/low pressure lines on my 91 Lumina Eurosport replaced. Some time after that I started losing power steering fluid and had a mechanic check the problem out and he suggested replacing the fluid lines again so that's what I had him do. However, this did not fix my problem and I continue to lose fluid. I have checked the pump on the engine and it doesn't appear to be leaking. The unit works fine as long as I keep it full of fluid but it leaks out slowly somewhere and I have not been able to determine where the leak is. The steering works fine otherwise. Can you possibly give me any idea what to do now or are there any trouble spots associated with this cars steering mechanism I need to know about. Any information would be most useful. Thank you!