As I understand it (someone correct me if I'm wrong), as springs get old and de-arch you loose caster. It also puts your drag link out of level. Now your truck is going to handle bad, and terrible on bumps. Now, you could put more shims in to correct the caster angle, but that doesn't fix the drag link problem.
You're correct. First the spring/caster issue:
If you look at the springs, you'll see the front axle mounts further toward the front and not in the center of the spring. As the spring relaxes, natural tendency of the way your axle is mounted is that it wants to relax in the center and not further up toward the front...that is to say it is going to relax about the rear point where the axle is connected to it. If the axle was mounted mid-spring this wouldn't be such a problem. As this happens, the axle flattens out and may even tilt forward when the spring goes upward (when a bump is hit) and caster angle is lost in the process. If the springs get really weak, the axle may tilt forward all the time.
As for the drag link, it is probably good to understand that even under the best of circumstances it is fundamentally not a great design. Think of it as the radius of a circle with the center point being the spot where it connects to the pitman arm (since this is the spot which won't go up and down). As the springs go up and down, you can easily appreciate the end of the drag link connected to your third arm will be pulling it in and pushing it back and pulling it in again as the end of the drag link travels through its little arc. It would have fewer problems if it was a foot longer in my opinion--since the end travel would be minimized the larger the circumference of our imaginary circle would be. As your springs weaken and the axle is essentially pushed upward, the tip of the drag link is pulled backward. The further up the axle goes, the further from the center point your sector gear will be in order to keep your truck going straight. This isn't a good thing at all. The design of your steering box is such that the center point should be the tightest point with points to the right and left of that becoming looser. If the front springs are very weak, your drag link will be pulling and pushing and pulling again as the springs go up/down. And this will cause your wheels to first go left, then straight, then left again when you hit a bump...and it doesn't take much of an irregularity in the road to cause your springs to go up/down.
From my experience, the worst of these factors for wandering is caster angle. A caster lower than the truck was designed to have will always cause wandering. A caster angle of close to zero can cause dangerous wandering. You can solve the worn spring caster angle problem with shims, but about the only way you can keep your sector gear in the middle is to build an adjustable drag link.
Jon
