The most probable cause is an internal leak in the master cylinder. You would not lose fluid in that case, but the pressure forces fluid past the piston, instead of down the pipes to the brakes, and your pedal (and piston) just keep on travelling to the floor.
I am not sure if it is possible whether wheel cylinders can also leak internally and not lose fluid.... usually they just leak out into the wheel..... but your wheel hissing might suggest having your wheel cylinders checked out...
There may be some kind of indication if you just take that rear drum off and take a look. Nothing should be wet.