Brakes keep failing after bleeding

We just bought this car and the brakes went all the way to the floor the lady said she would bleed them and drive it a while then they would need bleeding again.
So that’s what we did. The master cylinder was very low on fluid when we started and there was lots of air in the line but we got good pedal after bleeding them.
And sure enough after driving for a few days the brake light came on and just a little while later while driving the pedal went all the way down and wouldn’t pump back up, it just goes to the floor every time. The master cylinder is still full. So we are stumped. Help!