Confusing experience with brakes

[JohnMcL]

The setup is twin Wilwood m/cs with ap discs and calipers. On Sunday 18th June in +30C, drove 130 odd miles out, parked for lunch, then set off for home. During the return journey the brake pedal went soft with long travel. System could be pumped up but braking effect seemed reduced. So I put the car away and planned to sort it out this weekend.

Before starting work I drove up and down the lane to refresh my memory. Pedal is rock firm and braking effect is perfect.

Anyone got any ideas as to what is happening?

[JohnMcL]

Thanks guys. Plenty to check but I haven't found an obvious smoking gun yet. Onwards.