How do you know if you need a master cylinder?

If you are sitting at a light with your foot on the brake and you feel the brake pedal slowly sink to the floor but it pumps back up and the brake reservoir stays full, you have a bad master cylinder.

-OR, if the brake reservoir gradually lowers every day and you have to keep topping up the fluid. You check each wheel and they are all clean. -Then looking carefully at the master cylinder one day, you see the stains on the front of the booster - - -