Only enough for one good stop. 73 Duster

That sounds like the valve in the center of the brake booster is sticking. When this happens the booster diaphragm dose not return back because vacuum is still being applied after you release the pedal. Since this valve is internal to the booster and not serviceable, replacing the booster is the only option if this is your problem. But first check the vacuum port, hose and check valve for any restrictions. One way to do this is to remove the vacuum hose from the intake manifold and the brake booster. Using compressed air blow it through from the booster end of the hose.. It should blow freely. And should not blow through at all the other way.