My 55 wagon does this if/when I let it sit for a bit. Not usually an issue if I am driving it every day, but if I stay home all weekend, when I get in Monday morning the first brake press of the day goes all the way to the floor. After that it works as you would expect. I just make sure to pump it a few times before I start.
I just sort of assumed that some of the fluid drained back into the master cylinder.

-VTWilly