Power Steering Problems

[MACKL]

I just completed adding power steering to my truck and I only have power assist when I turn the wheel in one direction. When I go the opposite direction it is next to imposible to turn.
I pulled the box and pump off a donor parts truck 2 years ago. The po said it worked, but he lied about everything else so this may have been a tall tale too.
My logic says it is in the box. The pump simply supplys pressure and has not control over right or left. It acts like there is a control valve stuck inside the box that will not activate the assist when it is turned in both directions. In 26 years of turning wrenches, I've never been inside of a P/S gear box.

Anyone have any ideas of what could be causing this?

Thanks...Kelly