If the switch on the master cylinder is suppose to warn of a MC failure - would you know anyways by the brakes not working properly? Or does the light trigger before the MC fails - acting like a warning to eminent failure?

No, the switch pretty much doesn't tell you anything until the pedal has already sunk to the floor. BTDT, needed new underwear.

--DD

I thought the sole purpose of it was to remind you that after you replace the master and bleed everything that you need to press that button....it's a self serving button.