Brake Issue

If I were you I'd replace the master cylinder and see what happens. I had one go bad on the starting line of all places one time. It was a stock one. Brakes were fine all day, I staged the car and I was waiting for the tree to come down. I thought I felt the peal slowly move towards the floor as the tree came down. I left like normal, my opponent went red luckily so I decided to lift and make sure I had brakes and as I wasn't 100% sure if my pedal went soft or it was in my head. My pedal was WAY soft, like go to the floor, so I'd slowly pump it and on about every 5-6 pumps it would sorta work. I made it back to the staging lanes creeping along. I ended up having a .003 light, that got me the bye into the finals. I sat out the bye and had to run my sister in the finals. In between all that we checked things over and couldn't find any leaks or anything wrong. I think I dialed like 19.99 in the finals against here and had to idle when I staged as the brakes wouldn't hold. Made a slow pass in the finals that I lost, but ended up replacing the master cylinder and it fixed the issue. Not sure what the chances are both it a 1973 style master cylinder with 2 ports, so something took a crap, I'm still thankful to this day it I felt it on the starting line, because it would been ugly to find that out 1/4 mile later going about 115 mph than.