A spring would offer a fixed pressure in the failure mode (deceleartion is also dependent on tire loading and traction).

Now during normal brake application, the air pressure is adjusted to allow the spring to exert more or less pressure to adjust for the stopping power equired. The maximum pressure is fixed by the spring rate, but that can be far greater than the pressure you can apply to the system with your foot and what ever boost system is used.

Bob's system (that he should be getting a patent on) would just use electro-magnetics to adjust the applied force if it is implemented like the air brakes. or his system would just have the failsafe brakes and the motor/generators would do almost all the stopping. I think you would still need the friction brakes for faster stops and complete stop and hold situations.