I don't know exactly how your union is gonna interpret this, but I see how the union rep is trying to spin this -- she is trying to spin this matter into "more senior staff gets first pick in shifts", and by extension of priority shift picking, that also determines when the senior staff's vacation days are.
There is certainly logic behind that line of reasoning. IIRC, you were saying that previously, work hours were picked as a single, entire 40-hour block. Should you choose to continue fighting it, I think you should focus on this past, long standing practice of picking an entire 40-hour block and use it to dispute the union rep's past practice claim.
But if the union ruling determines that shift picking doesn't necessarily mean choosing single batch block of hours, then I think you'd be more or less out of luck.
-Lik |