Spongy brake pedal on 1997 Pontiac Grand Prix

I purchased the car recently used.I recently changed the pads on all 4 wheels. After changing brake pads, the brake pedal still travels farther than it should, almost to the floor. I bleed the brake lines, with no increase in pedal. There is no visible fluid leakage anywhere. What could be the problem?

You may have to do an ABS re-set. After brake work on most GM cars, this is a needed step. You need a GM capable ABS scanner to do this. Every time I even bleed or flush brakes on a GM car with ABS, I have to do an ABS re-set beacuse the pedal is very soft and goes to the floor.