We have a 1997 Sebring Convertible JXi that my son inherited from my dad. It is in decent shape with 60k miles. About two years ago the front brakes were replaced at a shop. We recently did the rear drums, shoes and cylinders. The pedal pumps up to firm with about 10 strokes with the ignition off, but as soon as you start the car and press the pedal, it goes to the floor with the slightest touch and a whooshing sound. Unable to lock up the brakes. We also replaced the master cylinder, with no change. For 20k miles the fronts look pretty grungy, and we may attempt to change those along with the calipers, as the bleeder valves will not open for the life of us. Any other thoughts on what would be causing this?