2003 Ford Taurus Brakes will not bleed

6 months ago my brake light came on and it appeared that I was low on fluid. I added fluid and the light went off. A week later it came on again, but the fluid was full. Thinking that the pads were worn, I had them check on the next oil change, but they were fine. As a result of some shimmy when applying the brakes, I decided to get a brake job this week. They reported that a rear caliper was bad and they replaced it. They bled the lines with the engine off and the pedal is stiff & high. As soon as the engine is turned on, the pedal sinks to the floor. The tried to reset the ABS computer with a scanner without result. They don't know what to do.