Brake pedal goes to floor there is fluid in reservoir

The master cylinder is in front of the driver's feet, just to the right of the left front wheel. When the leaking is in the middle of the vehicle, it is almost always from a rusted steel line going to one of the rear wheels. On some vehicles it is easier to unbolt the line at each end, then just replace it with new pieces of pre-manufactured line. Those are pretty inexpensive. On some vehicles it is almost impossible to remove the complete line. For those, we look for two rust-free areas where we can cut out the rusted section, install fittings, make double-flares, and then install a replacement line just in that one section.

Be aware that given the age of the vehicle, pushing the brake pedal all the way to the floor likely damaged the master cylinder from running the pistons and lip seals over the crud and corrosion that build up in the unused areas. You may need to replace it if you cannot get a solid pedal after the line is replaced and bled.