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I recently replaced my steering box with a rebuilt one, along with a new power steering cylinder, control valve and (GM) rag joint. I followed the instructions found on this forum for bleeding and balancing the system. The car drives straight with no pulling to either direction and the steering wheel is centered. The only problem is that it takes more effort to steer to the left than it does to the right. It is not terrible, but it is noticeable. Also, the steering does return to center by itself after a turn.
I was not very impressed with the movement of the box before I installed it as it seemed a bit "notchy". Is it possible that this can be adjusted out, or should I just send my original box (which I kept) out to a quality rebuilder?
I guess you looked everything over when you replaced the box and nothing jumped out as odd. Things can look good, but there can still be hidden problems. Maybe you could jack up the front end (under the spring pockets so the suspension is loaded), and turn the wheels by hand to see if it's tighter one side than the other, without the power steering helping out. I once had a bad tie rod end that did what yours is doing.
If you don't find anything binding or a bad steering component (link, idler arm, etc.), the most likely cause for your problem is a 'bad' P/S control valve. That valve delivers the necessary pressure to the right or left feed line to the P/S cylinder. And, if it worked OK before your latest effort, you can rule out the P/S cylinder. [It is not likely to be the cylinder, anyway.]
Yes, I read that it is a 'new' steering control valve. But if you don't find a mechanical reason for your problem, it's still likely to be defective.
I agree with the earlier reply that the valve is the most likely culprit. If you think your steering box might be the problem, jack up the front end, disconnect the slave cylinder from the frame mount (1 nut) and with the car off, turn the steering wheel from lock to lock. Make sure the loose slave cylinder isn't banging into anything. You should be able to tell if your steering box or something else is binding. If so, have the original box rebuilt or replace the offending component(s). Don't forget to replace the cotter pin on the slave cylinder mount. I've not seen it, but it's possible that a tie-rod end (or more likely idler arm) could bind in one direction.