I had a drum brake trailer from a different brand that locked up the brakes every once in a while and left flat spots all over the place. I finally figured it out after two different tires. I too thought it was the tires (goodyear marathons), finally figured it out when I started to take apart the brakes, drums, and bearings. Once I got down that road I did not have any further problems. And yes it was not torsion arm axle as well.