I know this may sound odd, and it is a different type of system. The rear brakes use fluid from pressure pump directly, modulated through the brake pedal, hence there is a direct path from the reservoir through the pump and out to the rear brakes. The front brakes are more conventional with the pump supplying boost pressure to the master cylinder which amplifies brake force from the pedal. Due to the design of the system, if the boost pump fails and the accumulator is depleted, there is no rear braking, only the front brakes in non-power mode.