About 10 years ago I filed a grievance for 20 hours pay. I was laid off from driving on a Friday (they sent me a message) and told to report to the preload and Twi the following Monday.
I show up and find the preload manager. He says sorry, but the package center has to give me a full weeks notice - You can not work for a week. Come back next Monday and I will work you.
I come back later that day and find my package manager. He says call the TWI manager to see if you can work. The TWI manager says come on in.
My package manager tells me "I guess you can only work 4 hours a day, sorry". I tell him that I am guaranteed 8, he shrugs his shoulders and walks off.
I find a steward. He says work 4 hours/day all week and file the grievance for 20 hours on Friday. I file, the next Monday the BA shows up, and my manager agrees to pay me 20 hours.
My manager then pulls me aside and says " You didn't have to file a grievance, I would have found you 8 hours of work every day. Next time just ask" . I decided to not argue about it, and just said "o.k.".
Nothing was ever said about it, no retaliation.
I have filed on excessive overtime (before the triple time rule), and an unjustified warning letter. Never heard anything about those. The BA (who has been replaced) was always "going to check on that".
Good Luck with yours.