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ABS question

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Hey gang this is not necessarily a Mack question but I figure I'd ask here as I've gotten great advice from this blog over the years.

I bought a used Trailstar dump, a 2017.  Previous owner just did the brakes, allegedly. Drums and shoes look ok.

When empty and slowing down, below maybe 15 or 12 mph, the truck starts jerking violently as if the ABS is kicking in.  Loaded it wasn't really noticeable.   It was so bad I unplugged the abs control unit.  I figured no jerking but no abs (until I can figure this problem out).  So now the brakes lock super easy on the front axle, both wheels, when empty......but the jerking is gone.  

What I'm guessing might be happening is the proportioning valve is not working and the ABS is actually sensing it.  I don't have diagnostic tools to check this stuff but I'm not sure that stuff would indicate a bad proportioning valve.



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Probably needs to be rebushed. Dump trailers often used Hendrickson HT 250 air suspension , they use a tri-acting bushing. What you describe is exactly what they do when those bushings need to be replaced .   That bushing does the job a torque rod would perform stopping the axle from wrapping up while braking. 

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