Or you could 1) talk to your union and 2) contact the Wage and Hour Division, the NLRB and the EBSA of the DOL. These only cost you a phone call versus hiring an attorney.
You should be asking your main union office to explain how and why the local rep was able to enter into an MOA without ratification by the membership. Of course if it's been done that way for over 20 years then you likely don't have a valid grievance, but never hurts to try.
Also, you're talking about a union plan, possibly a Taft-Hartley plan, which means some rules are likely different. I'm not going into the deep end of that pool.