Just need a confirmation on what I think is air in the brake system.

Well something arose today...a few times the brake pedal had almost zero pressure, only responding when depressed it all the way to the floor. Ususally right after this (when I got off the brake) pedal pressure would return. I also noticed a few times on the way home from work that sometimes it would offer ~%50 pressure.

I'm guessing these are symptoms of air in the system. The odd thing is I haven't touched the brakes for nearly 20k so I don't know how air may have entered the system. So my one question is...could this be something else like a bad master cylinder?

 

If you have no leaks, the first thing I would suspect is the master cylinder.