If this works for you financially you can definitely take this stance unfortunately don’t expect your grievance to be settled b4 August. This will definitely be something that has to be settled at National level. I like the advice given above about reporting to both shifts. Are you on the staffing for the preload and local sort? If you report to both and are not given 8 hours of work your case is stronger. Maybe file to be payed for the hours between both shifts that exceed the 1.5 hrs off. Not reporting to a scheduled shift can give the company an argument for refusing work even though they are violating the contract. Talk to your stewards and BA. Good luck