1973 Dart brake failiure

That happened last year when I purchased my Dart

It had sat for years inside and was in excellent condition when I brought it home

After I drove it for several miles on several occasions I noticed the pedal felt weak

After checking the fluid it was very low
I topped it off and drove a few more times
It stopped leaking after topping it a couple of times

My car has 4 wheel factory drums and has never been apart
It was full when I purchased because I checked it first hand
I’m guessing it developed a leak after I started using it some and I was getting ready to replace some seals or the master

But it has stopped leaking now but I still check it every time before I take it out to drive

Tommy