Leaking master cylinder

This is on the '70 GTO. The brakes have not been working and the pedal goes straight to the floor. I noticed there was no fluid in the rear of the MC so i added some (ignoring the question of why there was none), bled the brakes and the pedal seemed a bit harder, but weaker as we went. So we started it up and gave the brakes a shot, but there was no brakes.

Then I noticed leaking brake fluid from b/w the MC and booster and I assume that is the cause of air immediately getting into the system and causing there to be no brakes.

So what could be the cause of this leak or will I have to take it apart to see? Thanks

buy a new mc. the mc is bad. seal is gone or bore is pitted on the old mc.you may have to re center the proportioning valve or it may go back by it self .bench bleed the new mc put it on bleed out the brakes

sounds easy enough. Thanks Pat!