1996 Buick Park Avenue

I recently had my master cylinder replaced ($250) and was told by a reputable mechanic that the rest of the brake system looked to be in good repair. 2 days ago, I had to add brake fluid to the resevoir as it was low enough to kick the sensor on. This morning, my brakes became very squishy, and I had to again add more brake fluid. I do not see any obvious signs of a leak, but I also have rear drum brakes. Is it a possibility there could be a leak inside the drum or that the new master cylinder is bad?

Thanks for any assistance/advice.