I have to agree with the others. Check for leaks, most likely is a wheel cylinder. You can buy new cylinders at your mom & pop auto parts.
Bleeding procedure. You will need a new quart of DOT3 fluid, a clean funnel, a wrench to fit the bleed fitting, a piece of tube that fits snug over the ball on the bleed fitting, and a clear plastic bottle, I use a pop bottle, about 16 oz size.
Bleeding will be in this order: Right rear, left rear, right front, left front. You always go from the longest to the shortest run. You also need to have the brake drums installed.
Fill the master cylinder. Attach the hose to the bleeder valve, and stick the hose into the bottle. You will have to start with an inch or so of brake fluid in the bottle, and keep the hose end immersed. Have your lovely assistant push down on the brake pedal and hold it down. (sometimes 2 or 3 pumps are needed). With the pedal down, open the bleed valve, and when the fluid stops flowing close the valve with the pedal down. (Very important to keep from sucking air back into the system.) Repeat the pump and bleed until no more air bubbles blow out and the fluid is clean. Tighten the valve and move to the next wheel.
Be sure to keep fluid in the master cylinder or you will have to start all over again. (it helps if you can teach your lovely assistant how to check the fluid level and keep it filled. You will use most of the quart of fluid, and since the shelf life of opened brake fluid is terribly short, only keep the leftover fluid until your brakes are working properly.
Another pointer or two when working with brakes in not to contaminate the fluid. Don't use any oils or lubricants other than brake fluid. Keep the dirt out, and if you must clean something, use denatured alcohol. Flushing is accomplished by bleeding until clean fluid comes out the valve, which is also important. And you should adjust the brakes after you are through bleeding.
