If the brake pedal has become «soft», and with several successive clicks it becomes «harder», which means that air has entered the hydraulic brake actuator.
To remove air from the system, it is necessary to bleed the brakes. The procedure for bleeding the brakes is quite simple, but it is more convenient to carry it out with an assistant.
A detailed description of bleeding the brake system can be found in sec. 9 «Brake system» (see «Bleeding the hydraulic drive of the brake system»).