72 Duster Resurrection

Absolute cheapest way to go is probably rebuild the drum brakes, and flush out the lines. If on the cheap cheap, I'd at the very least pull the drums off and look at the pads to see if they have enough pad on them. if they look good, and theres no leaking at the wheel cylinders, i would open up all 4 brake bleeders and shoot compressed air from the master cylinder down through the lines to make sure they are clear, and blow out all the old fluid, then i would replace all 3 flex hoses since they are probably original and dry rotted. Refill the system and bleed it, and see whatcha got pedalwise. That is the absolute minimum. If theres no leaks, shoes are adjusted properly, and you still have a soft pedal, you will probably need a new master cylinder. While you have the front drums off , now would be the time to buy new front wheel seals, and clean and repack the front wheel bearings.