The master cylinder failing is a pretty unpredictable problem if that's what it is. What usually happens is one of the internal 'O' rings starts leaking so fluid pushes past the rings and drops the pressure until there is not enough pressure to push the pedal back against the helper spring. Since it's internal, there's no easy way to tell it's going to happen.
If it is the master cylinder, I'd ask him to take care of it and if he offered to split it that would be acceptable.
Good luck!
