The NACCO axle uses the same discs for both axle ends. I think maybe when the discs hang up you get a jolt at the drive hubs. You can check to see if the booster is holding the master cylinder under pressure. pump the brakes a couple of times and then loosen the bleeder. If the master cylinder has pressure on it you will see a lot of fluid coming out of the bleeder. The master cylinder will need replaced or kitted due to the wrong fluid.