Weird brake or suspension problems Peterbilt 379

Hi all. Want to say thank you in advance. I've learned a lot from this forum though this is my first post. I have a Peterbilt 379 dump truck that is making me tear my hair out. Some months ago it started pulling to the left when I would brake. We did the steer axle s cams and bushings and the problem corrected itself. Forward a couple months and it starts pulling left under braking again although this time was a little different. When the truck is empty it's really subtle. It felt more like something was shifting in the rear suspension because the steering wheel would stay straight and it almost seemed like the rear end would swing out to the right side. . Then when you're loaded with the drop axle down it does the same just more distinctly. If You punch the brakes you can definitely feel the rear end move out and the drives move to the shoulder and the truck tries to head into oncoming traffic. However all that said running the truck solo it is not bad to drive at all. It is easy to keep on the road. And honestly I wouldn't even think anything was that wrong with it being as it's an older truck and sometimes they seem to have their little quirks.

When the truck is really bad and unacceptable is when I put the tag trailer behind it and our 18 ton excavator on it. There is a lot of hitch weight and not much weight on the steer axle. If you have to panic break the truck will dart to the left. All the while the steering wheel stays dead straight. It feels like the whole rear end of the truck is swinging to the right and the truck just heads off to the left. We've changed air valves, torque rods, s-cams and bushings, slack adjusters. Probably a few other little things I'm not remembering. the suspension is Hendrickson walking beam and it is quite recently rebuilt. Just had it into the shop again yesterday and the mechanic went over the rear suspension very closely and said it's all in nice shape. The truck has newer brakes all around. The shoes and drums are in good shape. I am stumped. Do you have any good ideas?

 

I'm positive it isn't in the steer axle. When the front s cams went bad the steering wheel would turn/pull when you hit the brakes. How it is now the steering wheel stays straight. Something is going on in the drives. It's Hendrickson spring-beam suspension. When you turn with a load the axles do "walk" side to side around 2-4 inches per axle, but they seem to do that on every spring-beam truck I've seen. Am I wrong here? Is any side play too much? I think the rebuild was very thorough. From what I understand they did all the bushings. It has had about 80,000 miles since the rebuild but that doesn't seem like nearly enough to be worn out. The shop that did it is really good in general, so I'm inclined to think they did a good job on this one. Immediately after the rebuild the trucked stopped nice and straight.

The drop axle is a steerable axle but it doesn't seem to make much difference whether it's up or down. When we haul the excavator we run the axle up as there wouldn't be enough weight on the steer axle to run it down.

 

No leaking wheel seals. I've been over this truck front to back with a fine toothed comb. All the obvious things are in good working order. Today I moved the excavator and was experimenting with pulling the Johnson bar while I brake with the foot pedal to see if it straightened out. It seemed to help somewhat. Not sure if that's helpful or not.