It is very possible for fluid to leak past the master cylinder piston, and not let air in while it's doing it. If the pedal keeps going to the floor with pressure on it, it's probably the master.

 

But then the brake pedal would be erratic in the opposite direction and would not hold brake pressure at any time. A hole or tear in the master cylinder plunger isnt going to act erratic.
I'm not saying I'm right on this diagnosis but just exploring other options.....

 

Might actually be two problems. Found a tech with a good scan tool. We were able to monitor the ABS while driving. Brakes didn't fail on test drive, but we could see the front left wheel speed sensor dropping to zero regularly, while the front right and rear sensors matched the Speedo. Theory is that sensor goes to zero, and abs releases the brakes, thinking it's locked up. I think that's supposed to trip the dash light, but it doesn't. Trying to decide on new hub or just sensor. Haven't had time to jack it up and check hub.
New master cylinder en route also, doesn't feel right, but may not be only problem.
Thanks for the help, I'll post back whatever ends up fixing it.