Recommendation. If the brake system is in good condition and no air has entered the hydraulic drive, you should not drain all the old fluid from the system. It is more correct to replace the liquid by displacing it with new liquid. Also, to reduce the time required to complete the job, use a rubber bulb or syringe to remove as much old fluid as possible from the master brake cylinder reservoir.
Without starting bleeding, fill the reservoir with new brake fluid. We bleed each circuit of the working brake system until new brake fluid starts to come out of the fittings (she is lighter).