Yes you will have to bleed the rear brakes,But more importantly ,where did the fluid go?
You may have a bad line or caliper leaking,fluid just dont evaporate in a closed system.
To bleed the brakes you will have to pump up the peddle,and release the bleeder as you hold the peddle,and repeat untell you get the end results.
I always used a mighty vac bleeder,but bought a injector that worked awsome when I did my brake lines.I just shot thr brake fluid from the caliper up and instantly had full peddle and handle.