The master cylinder on my 71 Buick failed at the end of a 150 mile trip. Fortunately I was 2 blocks from home and nobody else was in the intersection. There was a little braking left, enough to get home and then had it towed to the shop. There was no warning and the brake light only came on as the pedal sank to the floor.
This is not something you want to mess with, please get it fixed for your own sake.
Ed B.