Brake pedal goes to the floor when first started

History: Eight months ago had pedal go quickly and easily to floor after just starting up. Never while driving. About three or four times it occurred.I was told that the fluid was low by Napa and my mechanic replaced some lines. But it happened twice even after he did! Last six months, all is fine! Last evening, drove and all was fine. Today, started her up, red brake light on and brake pedal goes easily to floor. Tried to pump it, but no luck. Checked fluid, looked fine, not low at all. I am totally confused! Bought this car from an elderly couple where it sat in a garage for many years. Could you give me some idea as to what's going on?