Rear master cylinder diaphragm

Does anyone know the purpose of the rubber diaphragm that goes into the rear master cylinder with a kz with disc brakes? I noticed it when I was bleeding the rear, it seemed I got more fluid flowing through the reservoir if I took it out. How do you re-install it if the reservoir is full of fluid? Drain the excess and just push it in? Thanks

Every brake reservoir has one, most act partly as a seal. I believe they are meant to allow for changes in air volume as the fluid goes in and out or when the level drops from pad wear without outside air directly contacting the fluid since it is hygroscopic.

I'm an idiot, that's exactly what it is, but what had happened is it got pushed down into the bottom of the plastic cup. I took it out and pushed it back to shape.