Over the years I've always seen the reaction disc problem described as one where the brakes are either full on or full off. I think that's why it's called a reaction disc. I feel compelled to say that a fallen reaction disc causing the pedal to slowly sink to the floor just does not make sense, physically, mechanically, hydraulically, or any which way. The actions of doing the work to take things apart, find the disc, glue it back in to place, and rebleed the brakes might be leading to better pedal function and brake operation, but there's no direct rational reason for the missing disc to allow the master cylinder piston to travel the length of the cylinder without creating pressure..
I feel better now. Magic makes me nervous. Be gone evil spirits!!
Edit - unless the missing disc causes the MC rod to "cokc", causing the piston seals to leak. Maybe that's it. Indirect effect.