Well, up to this point I haven't been thinking about the power steering valve (in the box). If all is right with the world, when your sector gear is in the center position (right in the middle) then your valve should allow the power steering pump to send fluid to the power steering valve, divert it without using it for any work and send it back to the pump. It should do this infinitely unless the valve isn't working correctly. When you begin to turn your steering wheel (say to the right), the valve moves and opens channels to allow the pump to send fluid in to push and assist your effort to turn the wheels. When the turn is finished and you're going straight again, your pump and valve should return to the job of just recirculating the fluid. Since a picture is worth 1000 words, I took a moment to look on Google to find a simple explanation:
http://www.agcoauto.com/content/news/p2_articleid/214I think what your friend is saying is the valve in your steering box is faulty. If this happens, fluid may continue to go where it isn't supposed to be when everything is centered. If this happens it is like somebody grabbing the steering wheel and trying to turn it in that direction while you're trying to drive straight. In really severe cases, the car may try to steer itself with vigor. But problems like this are not all that common.
If your friend is on the right track (or not) can be learned by setting everything in the center, jacking up the car and starting the engine. In my experience, nearly 100 times out of 100 times if the valve or the seals in the valve are not working, the steering wheel will start trying to turn. If it doesn't, we go back to some adjustment (or other) problem. If you want to disconnect your drag link, you can do that, too. Just set the steering wheel in dead center and start the engine. You don't have to jack the car up in this case. If the wheel turns to the right, you have caught your culprit.
To answer your question of why does it want to pull when straight ahead is set at dead center? The pump's job is always to pump fluid and if the valve isn't working or if it for some reason is not at its center, it can let the fluid trickle over to the place where it pushes against the power steering piston. So then you have the invisible hand down there steering for you...
