Stop driving it. A pedal that sinks is either old fluid boiling, a swollen flexible hose holding pressure, or a master cylinder past its seals. All three are cheap next to the alternative, and all three get worse with heat.
Bring it in, or call and we will come for it inside 15 km. We pressure-test the system, look at the fluid's boiling point with a tester rather than at its colour, and tell you which of the three it is.