Never worked on those machines, but dad had to work on the steering on a couple of "cheap" cadets about 10 years ago that used plastic bushings on the steering shafts/sector---something in there wore out premature. I think MTD replaced with bronze. Are you using plain bronze or oilite (oil impregnated)? I think the later would be better, but can't see how either could hurt.
I don't get why plastic (most likely nylon) bushings fail in some applications. I've got idler sprockets on a 30 year old corn planter that are made out of nylon. They are worn but still work fine. Seen plenty of bushings hold up, seen them wear in 1/2 as well. I think some applications don't have the bushing securely held in place and if it wiggles around, they are more prone to fail. That's my guess. Wish I could be of actual help on your machine instead of just rambling about plastic parts
