Master cylinders go bad from the seals failing. But it can be caused by using the wrong brake fluid or other contaminants getting into the brake fluid.

Brake fluid sucks up moisture like no tomorrow and over time it can become very heavily contaminated with water. This can lead to rust and corrosion brake lines and such.

For the pedal to go to the floor with no fluid loss the fluid has to get around the piston in the master cylinder. The piston has a rubber seal on the outside of it that seals it against the housing the piston travels in. If the seal shrinks, gets a tear or micro holes in it the brake fluid can seep around the piston allowing the pedal to go the floor. Pumping the pedal up gets fluid back on the correct side of the piston restoring the brakes to normal use. But it will leak down over and over and the only fix is to replace or rebuild the master cylinder.