What is the best and easiest way to check if backing plates are bent? Make a lathe adapter out of an axle retainer and measure runout seems to be the most accurate but tedious. Taking one off the car, place in a drum and measure height?
One you determine how non-flat it is, how do you straighten them?
I don't see availability of new backing plates (rear) online.
Keep in mind disc brakes are not an option in a vintage applications.