Anyone who works Saturdays as pre-load has witnessed that Saturday’s are the worst days to load. Management, has half the staff of tue-fri. Monday’s are also a disaster. As half are off Mon or half are off Tue. But the pre-loaders are still required to load the same amount of packages. You get 5-7 trucks, normally it’s 3-4. The packages come down at the same rate as if everybody was there, but of course it’s only half staffed. Yesterday 8 people called or no showed, so it’s less than half. Management should be required to train loaders on how to load 5-7 trucks, with 250 or more packages in each truck. They won’t train us, we’ve asked, they cant load that, and they know it. The union has failed here as well. The union could careless about pre-loaders. 90% of stewards are drivers, and you ask them about pre-load, they have no clue.
If you need more part time stewards talk to your local's principal officer, or at least your BA, about adding another steward for your sort, then put your name on the list. Then, go to the effort of learning the contract, zeroing in on part time issues. I know enough to help out part timers, and when a specific issue comes up, I'll research it, talk to my BA or another Steward to figure out the best way to handle the problem. But I don't spend a lot of time specializing on part time issues beyond that.
As for how to handle your situation, work at a pace that you are comfortable with, if packages pile up and begin to block egress (which is a legitimate safety no-no), stop the belt, or let packages go by, until you can get your area clear. If preload is still going when the drivers show up because they aren't staffing properly, the drivers will have to step up and start loading after their start time, or will need to punch in early. A few Saturdays of that should send a message that staffing needs to be fixed.
If you get yelled at or threatened with discipline for working at a safe pace that you are comfortable with, or stopping the belt when necessary, that is when you file grievances for harassment. That's how it goes sometimes. It may not be the most comfortable thing to do, and you may not like the idea of becoming a target, but if you get the rest of your sort on board, no one has to be singled out.