Getting my *** handed to me.. these damm brakes

I have and am chasing a similar issue. Air getting in. But no fluid leak out. I have read that this is possible with brake cylinders and/or master cylinder. Mine has the front disc brakes disk. And rear cylinder drum breaks. I read that it's possible the cylinder of either master or cylinder to move past seal protection that can allow air in. But once pressure is applied close to prevent fluid leakage? That this occurs using break cylinders or master cylinders that have been rebuilt perhaps one too many times?

I know it's throwing darts as speculation. But makes some sense?