Making the best of a bad situation

As mentioned, with the engine off, take up any free play by using your hand to feel for any. You only want 1/8" of free play movement in the pedal. If you still have a low pedal, but, it's hard and the shoes adjustment is correct, there's a mismatch there somewhere. I'd be tempted to get rid of the booster and run a stock '67-up drum type master and a stock push rod. If the pedal is soft/spongy, there's still some air in the system. Of course, you could just go with a disc brake system and really improve the situation! :) On a side note, the factory setup in the '66 and older models with a single stage master cylinder, do have a return spring. Not like the one you have, but, it's a weak wound spring and rubber boot that goes around the pushrod. It's just enough of a return spring to turn the brake light switch off.