Hard brake pedal with very little braking ability?

Because you provided the necessary information, your issue is simple to diagnose, and fix.

You have a vacuum line that is either disconnected or leaking (cracked). I suspect it is the line leading to your brake system vacuum booster. (where it connects to the booster, or at the engine)

Your booster is located directly behind the master cylinder (where you put in the brake fluid). You'll see a black rubber hose going into the round booster. Check that connection and trace the hose back towards the engine. You should find your leak. (also, you'll probably hear the hissing sound of the air leaking)

It is possible that your vacuum leak is located somewhere else. If this is the case, it's just as simple to find. Just check each vacuum hose on your engine and their connections.

If I've helped you, please take the time to vote for me, as my reputation pays my bills. Thanx, Ernest.