Concur with Boru, a soft brake pedal going to the floor is related to the Normal Brakes (NB) system, not the ABS. The ABS is a complete different system, but it relates to the NB in our cars in mysterious ways.
My experience with such symptom has been a failing Master Cylinder (MC) unable to generate pressure due to deteriorated seals in the piston. If you are loosing brake fluid at the MC reservoir then you should try to track where is it going. If you see a trail of dirt or fluid under the MC, on the face of the Brake Booster (BB), BEWARE! It could mean fluid is being forced inside the BB and with the age of a Legend, it is very likely the brake fluid will also fail the BB. A failed caliper is another possibility, this can be checked by looking inside the tires, a wet rim is a good sign of a failed one. If you park the vehicle in the same spot every day, check for a brake fluid dirt spot for the leak will be in a brake line nearby.
Whatever you do, don't ever try to outsmart a Brake problem, and the soft brake pedal is a brake problem. It can be a very expensive and tragic proposition. Take it to a reputable mechanic, or get a knowledgeable person to properly diagnose it and fix it ASAP.