Re: @Steven Jones
Actually the assistance is mechanical/hydraulic too, not electronic. It works from the vacuum of the inlet manifold of the engine, which operates a servo to assist the pedal going down. It does not work "when the ignition is on", only when the engine is running, and has nothing to do with electronics. The reason brakes still work when the engine is off is because the brake pedal is hydraulically attached to the brake calliper, and the servo only assists, it does not actually control the brake calliper as an electronic system does. The same with ABS, it just temporarily overrides the hydraulics but never disconnects the pedal from the brakes.
It used to be the case (not sure if it still is) that the main brakes and the handbrake had to use a different technology to work each. I.e, you couldn't have hydraulic main brakes and hydraulic handbrake. You couldn't have electronic main brakes and electronic handbrake. I'm not sure if there are any secondary/mechanical backups for the controls in a aircraft, but I would sure hope so. Software bugs are a bitch.