From what you described, I think your saying the pedals are closer (less free-play) to you once the car is warmed up. If so I'd suspect water. You may also take a look at the line routing around the headers and if you can add a heat shield below the master cylinders. If - like most Cobra the headers are less than a foot away.
The next to last paragraph in this article describes the pedal issue.
https://www.good-guys.com/hotnews/te...d-brake-fluid/