Hi, the problem is usually the brake wheel cylinder inside. Put the car in park but do not set the brake. Jack up the wheel and remove the wheel and drum. At the top of the hub assembly you will see a cylinder between the tops of the shoes with rubber boots on each side. If fluid leaks out when you pull back the edge of either boot, the cylinder is leaking and must be replaced.
Purchase a new cylinder and also some brake fluid if you don't have any on hand. Check your reservoir. Hopefully it is not empty, or you would have drawn air into the system. Fill up the reservoir before replacing the wheel cylinder.
To replace, use a vice grip plier to remove the return spring that holds the shoe tops together, then pull the shoe tops away from the cylinder. Place something under the hub to catch the fluid that will leak during the replacement.
Loosen the hydraulic fitting on the back of the hub using a line wrench of the correct size, then remove the 2 bolts holding the cylinder to the hub (see picture below). Have the new cylinder handy and disconnect the line from the old cylinder. Quickly remove the old cylinder and put in the new one. Screw the line fitting on finger tight and then replace the bolts. Replace the return spring between the shoes. Tighten the line and open the bleeder valve above the fitting about one turn. The line may gravity bleed the air out. Give it a few minutes. If clear fluid starts to leak out and no bubbles, tighten the bleeder and reassemble the wheel. If it does not gravity bleed, replace the drum and have someone assist with a pressure bleed. With the bleeder closed, have the assistant pump the brake pedal and then hold it down while you open and close the bleeder. They should not let up on the pedal until you have closed the bleeder. repeat this process of ejecting air and fluid until only fluid comes out. Then replace the wheel and let the car down.
Please let me know if you have any questions, and thanks for using FixYa.