Ghost Brakes

Hey, I am new to this site so I hope I am posting in an appropriate place. I have a 1985 AMC Eagle Ltd Wagon. He is having a small brake problem. Every so often, his braking gets squishy even though he has plenty of fluid. I am using 3 dot and the master cylinder does not change in it's fluid level, so I am pretty sure that there are no leaks. Does any one know what might be happening?

Must be getting air in the system somewhere. It could be a leaking seal at a wheel cylinder, or a soft flex line. Whatever it is, find it quick and fix it, brakes are nothing to ignore!