Unfortunately, an attorney will not be able to help you. You must file a discrimination complaint with the EEOC 180 to 300 days after the date of the violation. The time does not stop being counted just because your union has offered to mediate the problem. Your union may be able to get you into mediation, but if the mediation is not successful, you will not be able to threaten that you will then file a complaint with the EEOC.
Unfortunately, this situation is rather common, especially in shops where the union has a close relationship with the company. By stalling the mediation until past the time when the employee is allowed to file a discrimination complaint, the company has little incentive to resolve the discrimination issue.
If a union employee feels that he or she has a valid discrimination complaint, he or she should have an attorney file the complaint within the required time period. Then if the mediation fails, the union employee will still be able to continue his or her EEOC complaint against the company.