Brake issue 1998 wagon

Brakes just started going to the floor and not really stopping the car. Brake fluid is full. Didn't know if these vehicles have a proportioning valve that could be causing a problem or if master cylinder might be out????

First thing to do is completely inspect the brake system for any leaks. The reservoir may not be empty.......yet. Rear wheel cylinders can be a likely suspect!! If no leaks are found than the master cylinder could be a suspect.