I work a midnight sort and know a few PT cover drivers who were on my shift. The way I understand it is that a PT cover driver reports to the sort they were on when they went cover driver, but midnight sorts conflict with the DOT time off requirements so unless they know they won't be needed to drive for more than one day they just take the day off. For the most part they drive and maybe a couple times during the year do we see them on the midnight sort, but every location is different. If you have concerns talk to your steward, they can give you a better answer for your building than we can.