As Ted says replace the wheel cyclinders on both wheels - even though only one is leaking - it will require removing the brake drums and stripping down the brakes to gain access to the cylinders - clamp the flexible pipe supplying the rear brakes to prevent loss of fluid - disconnect the brake pipe from the rear of the cylinder ( if it haven't completey rusted solid )- install the new ones - re-connect the brake pipes - rebuild and adjust the brakes and then bleed the brakeing system.
If you are not sure or are not competent - get a mech head to do it for you - better safe than sorry.