If you mount a drum and hub brake lathe and put a dial indicator on the surface where the shoes make contact, the amount of movement as indicated by the dial indicator is the runout. On trucks with air brakes that have no backing plate, runout can be checked on the vehicle. Runout is caused by drums warping or by irregular wear which is more common now with non asbestos linings. We have drums to get spots so hard they will not cut on a lathe. We use a grinder attached to the cutting head to true them.
