Sounds like you may have one of several problems, and perhaps a mix of them. Have you bled the system? If there's air in the system, you will have a low pedal and ineffective braking. Could also be a leak somewhere in the system, master cylinder, hose, steel line, wheel cylinders are all suspect. I would advise pulling al 4 wheels and drums to check for cylinder condition, plus check all hoses, lines, and the master cylinder as well. Check the bleeder screws to make sure they come loose easily. If not, soak them with B'Laster or Kroil, then try again. Check the condition of the shoes. Look for worn/fluid soaked linings. If worn or soaked, replacemnt will be required. Same goes for any leaking component, either replace or rebuild, dependng on condition.
If everyhting checks out, then try bleeding the system. I use the following method. 1, starting at the furthest away from the master cylinder (check the line routing, as it could be either the left or right rear, depending on line layout), with a helper to pump the pedal, loosen the bleeder, ho;d your finger over the bleeder enough to amke a loose seal. Then have the helper pump the pedal gently. Your finger will prevent air from being sucked back into the system. Maintain pumping until fluid comes out of the line, then repeat with the remaining bleeders, going to the next furthest and ending with the closest. Remember to refill the master cylinder after each wheel cylinder beedling to keep it from running dry. You may need to refill more often, especialy with the first one. After doing that, repeat the bleeding sequence, with a slight difference. Instead of holding a finger over the bleeder, keep it closed as the helper pumps the pedal. Have them pump the pedal about 5-6 times, hold pressure, then open the bleeder. Observe the stream, looking for bubbles. Repeat about 3 times, or until there is no air in the stream. Then repeat for the remaining wheel cylniders, again, refilling the master cylinder as you go. Remember to replace the cap after filling. Using this method should result in a good, solid pedal. I use this exact method in my job as an ASE Certified Brake and Alignment mechanic.
