The rear brakes aren't preloaded. The spring actually pushes the pivot bar back, along with the levers to release the brakes.
The pivot bar runs on holes in the frame and the "bearings" are just doublers for the frame. They do support the bar, but are steel-on-steel. A little oil there can help the bar pivot freely. Not much else you can do.
I don't think that the pivot bar is reproduced.
