Sounds like your center isn't abiding by the contract, then. Specifically, ...
ARTICLE 22. PART-TIME EMPLOYEES
....
Section 4.
Part-time employees with six (6) months or more seniority shall have the right to place their name on the list of employees waiting to be moved to a preferred job within their building. Such preferred jobs shall include, but not be limited to: Preload, Sorter, Clerical, Irregular Train, Designated Responder, Carwasher, Loader and Unloader. Employees do not have the right to select any specific unit, load or workstation unless a prior past practice has been established...
Now, in my center clerks are in a different Local. But Brownboss, despite his very annoying unreadability (wtf...is he on the web using his phone?), seems to have a right to the pt clerk work, if it exists.