Here's one that might stump you

[Gone in 60]

I’ve had the opposite problem as MMC with my car, which shares the same fluid for clutch and brake. If the car is parked outside on a very hot day, mid-nineties at least, the clutch pedal is very stiff and the brakes a bit more grabby than normal.

I agree with a master cylinder issue. I had the same problem on an old Nighthawk. Rebuilt the master myself and replaced the old rubber brake hose with steel and the problem went away.