Providing that the brakes are operated by a servo that you control, and there is no trickery inside the car (sensors, ABS, etc.), then they are legal. I can't understand why you would want to do it as the additional load from braking will limit the load the tyres can carry for steering.
The front tyres are the narrowest so they can't take much braking force before locking up, whereas the rear tyres are wider and have more weight over them so they can brake much harder without locking up - and we already have brakes on the rear axle through the motor.
But since it is there, and it can be done, why not?
