You gotta hate jobs like that . if you open up the swing panel with MLB on it you will see two LED's B and BE on the terminal rack , these indicate the brake switch state . if your lucky enough to be watching those when the fault occurs then that may give it away . you can also have issues with the drive LVIB itself . in my experience if its the brake switches it will return and get progressively worse . if you do need to change them they are a easy swap out , 7 mm wrench and long nosed pliers make it easier . I had a fault on a unit several months ago that I could watch for hours on end and the minute I left would happen again , trapping people but showing no fault by time I arrived. I spent 7 straight hours sat watching the controller putting in random calls , had lunch and a Thermos of coffee in the machine room , after 7 hours I needed the bathroom and couldn't hang on any longer , the bathroom was just far enough away that I could hear the alarm bell ringing just as I was washing my hands . had corrected by the time I got back to machine room . one of those days you wish you took another career path