I am having brake trouble out of my 2004 Ford Expedition.?

The ABS light comes on every now and again but the main issue is the actual braking. On occasion when the brake pedal is depressed it feels as if there is air in the lines and the vehicle does not want to stop, almost a spongy feel but worse. Since this has started we have had the brakes replaced and bleed, and also had the codes read. According to the Ford dealer the code read a bad brake sensor that costs $200+. We replaced the bad sensor and the problem and light is still there. Does anyone have any advise?