2 wheel cylinders was normal for front brakes before the arrival of discs. It allows both shoes to be leading. Now that discs are up front we find on some rear drums the same 2 wheel cylinders to make both rear shoes leading. The OP should have squared away the rear brakes, which are much more complicated than the front, before messing with the master cylinder. Any shoes contaminated with brake fluid are useless and should be replaced. At this point in the task you should be looking at a complete rear brake replacement, other than the drums, and they may need to be turned.
