If you are paid every other week, there will be an extra paycheck in some years. The Company can legally argue they are paying you at the same salary rate even though they are reducing your paycheck to compensate for the additional check. I'm assuming you have a CBA in place and I'll bet it doesn't specifically address this issue. The force of "past practice" may be something to bring to the table but I wouldn't pursue it if your union's counsel suggests it will fail. All you'll do is get the taxpayers rallied against your cause.