Steering noise

I recently replaced the steering gear box on my 97 Safari, and I still hear a hissing noise when making turns. The noise appears to be coming from below the dash board. I've bled the system of any air, and have no leaks. The steering does get a little stiff from time to time. The hydro-boost unit looks easy enough to replace, but it's very expensive. If anyone could help, I'd really appreciate it. Thanks.