For the most part, my brake pedal is absolutely solid. But at least once a day, I'll hit the pedal and it will head for the floor. The pressure comes back once I pump the pedal, and once it has pressure, I can stand on the pedal without it descending. I have to fill up the reservoir every two months or so, but I don't find any leaks at the wheels or in the lines.

I'm fairly sure I know the answer. But before I replace the m/c only to find out it was something else entirely (it wouldn't be the first time), thought I'd ask for a second opinion.

Thanks all.