There is a discussion concerning so called 'brake-by-wire' system on the
Autosport forums, and I found especially interesting comments of user "TC3000" (not sure if he is active also here on F1T). Nevertheless, below are his thoughts on how this braking system would actually work.
When the "brake-by-wire" system activates, they (from what I get out of the
interview) will close the
Isolation Valve. This will disconnect the rear hydraulic circuit from the driver input (master cylinder) and the "controller" takes over. They would open the
Dump Valve.
What is here in this graphic shown as the
Accumulator would not only be a passive spring, but a movable piston on the other side of the spring too (a pump), this will give them the option to reduce or increase the pressure in the rear circuit "at will", in their effort to match/mate the two braking torque generating devices (the MGU-K and the rear hydraulic brakes) together, so that they achieve the desired overall torque.
The controller will not only need to balance with the MGU-K, but also still "listen" to what the driver wants, because the driver will keep modulating the pedal to control the front tyres and hold them close to the "lock-up" threshold. The drivers are used to the fact that front and rear act together, being operated by the same pedal. Now, the rear is "disconnected" from his foot, and has an additional link in the "chain of command". This link (the controller) can/will cause some slight delays, and the drivers will need to take this into consideration. Even if everything works "well" (let's not say perfect), it will need a little bit getting used to.
Some will remember all the discussions about Hamilton and his Brembo vs. CI brake feel issues. It's very real, not only for Hamilton. Some drivers are less sensitive to these things than others, but I'm sure every driver will at least notice it. Now, if the "controller" has a "mind of its own", then it becomes even more interesting.
Controller is balancing the braking work between the hydraulic brake circuit and the MGU-K. The controller will and can supply it's own (brake) pressure to do so - just like the pumping unit in an ABS block does - the driver is "disconnected" from this. When they, close the rear circuit, they "cut-off" the "elasticities" (line and caliper expansion and fluid compressibility) of the rear, effectively increasing the stiffness of the total brake system. We talk very small movements here, but the drivers are very sensitive to this.
Under normal conditions, driver presses the pedal, this will generate pressure in the two master cylinders, this pressure is then feed into the front and rear circuits. Before any retardation is developed from the brakes, this pressure needs to build up in the calipers. Along the whole path, are many things which will expand a bit, taking up some fluid volume (the give or slack of the system), you first need to take these things out, before pressure at the caliper rises, then the caliper flexes a bit in it's self etc.
All these small "imperfections" cause a tiny amount of pedal movement, but it makes it also possible and easier for the driver to "modulate" the brakes. The hydraulic circuit acts like a "spring". You have two springs, the front and the rear circuit, now if they "cut/shut off" the rear circuit, they change the spring rate of the complete brake system. The pedal will feel stiffer. And the driver still needs to make tiny adjustments to control the front wheels, but now the overall pedal feel will change depedning on the condition of the rear circuit (if it is connected or if it is disconnected because ERS-K is recouping energy).
Not that old 
