We used to do a 4x2 unit on local work which had one rusty disc on the front due to a previously overheated brake. It never really cleaned up through normal use and I pointed it out the tester at MOT. I told him they owner won’t replace the discs because it was only local, he said " we’ll see on the brake rollers, generally rusty discs have better grip". I was a bit surprised at him saying that, it sailed through the brake test and we never did change them.