Is it earthed through the loom or a strap/wire to the chassis somewhere?
I am chasing a similar problem. 95% of the rime my brakes are perfect. Nice solid pedal. Then from time to time the pedal is really soft and goes to the floor and i have to pump it up before i hit something.
Scary.
It does seem to happen when i am doing full lock 3 point turns etc when i hit the brakes its soft. But then is fine.
So definitely not bleeding brakes. The ABS light has been on for about 2 years but its not the failed sensor. If anything i can only track it down to since i removed/reinstalled the engine and sprayed the bay so an earth problem may explain it. Hell the silly slutch cable tenaioning up could be affecting the earth...obviously guessing here