I know how you feel.
I have more seniority than most of the other preloaders in my building and wondered if I could choose where and/or what particular job I wanted to do in the area of the building that I work in.
Just by coincidence, as I was skimming through the new union book, I saw a section on that very issue.
It said something about the employee with more seniority could request a specific job and, if necessary, put their name on a waiting list to perform that job.
So, I asked the driver in my building (who is our Local's representative for our building only) about the section I read and told him the specific job I wanted.
He went and discussed it with the Supervisor, and the Supervisor told him that our building just doesn't do it that way and never has or will. Supervisor said he, and he alone, decides who will work where and do what in HIS building.
So, I guess what I'm saying is that even though it's written in the union procedures book, each building's Supervisor can run things (within reason) the way they want to.