ABS problem in Peterbilt 579

Hello, one of our trucks has an issue with his ABS system when he starts his truck the ABS light on the trailer will come on for a couple of seconds and then go out which is normal. But if he stops for any reason(kills his truck) the ABS light on the trailer won't come again. My driver has been told this is not the way the ABS system should work, that the ABS light on the trailer should come on again for a couple of seconds everytime he starts his truck. I am agreeing with him on this issue but do not know exactly what is causing this issue. one other thing which makes me think there is some problem in the ABS system in the truck, is my driver can unplug the pigtail from the trailer then plug it back in, then start his truck and the ABS light on his trailer lights up for a couple of seconds. Does anyone have a good idea what is causing this? Regards, John Robinson