Anytime you have a pedal that is soft I would say it is a master cylinder failing. But it should happen all the time.
The interesting thing is that it happens when it is hot. What does the fluid look like? Is it very dark in color? It could be that the fluid has a high water content which might explain the soft pedal when it gets hot.
If the brake fluid has water in it there are going to be other problems as well. There will be components failing from rust.
I would have someone check the brakes and the condition of the brake fluid. It would be helpful for the person checking it to have it on a hot day so he can experience the soft pedal.