The Seniority System is set up by the Union. Some locals do it differently than others. If you're corectly stating the situation, then 804 apparently does it fairly by Company Seniority (your total years with the Company), rather than keeping full-time years seperate from part-time years.
In my Local, there are two completely seperate Seniority Lists, one for full-timers, one for part-timers. There's also a rule that any full-timer outranks all part-timers!!!
We have part-timers in their 37th year of Seniority! But if a New Hire full-timer waltzes in off the street and makes his 30 days, that new hire has more Seniority than even those long-time part-timers. All those years of paying full-time dues and initiation fees, and walking the picket lines in '76 and '94 and '97, mean nothing for bidding, bumping and layoff purposes. A part-timer that bids a regular driver job, or bids a 22.3 combo job goes to the bottom of the full-time Seniority List.
This, and a bunch of other anti-part-timer policies, is why part-timers don't get involved in the Union.