Brake pedal travel on 1997 Saturn SL1

My brake pedal sometimes travels all the way to the floor. The brakeware on the pads is about 50%. The car as you can see is very old with lots of miles on it. What could the problem be? Vacuum system/Master cylinder/ ABS system?

Asked by for the 1997 Saturn SL1
check the master cyl.put steady pressure on the brake pedal - if pedal sinks down any replace master.