94 PA brake troubles

[W1234]

Well, where to start?
Last year, May '03, I had a major brake failure and had a ton of stuff replaced, drums, rotors, calipers, pads, shoes... etc. Everything but the master cylinder. Sure enough, last month, My car started leaking a mystery fluid and the brake light came on. I took it in and had a master cylinder replaced. It ran fine until this morning when I pushed the pedal and it sunk back way to far, almost to the ground. The car still stopped, the brake pedal just went furthur than usual. I took it back to the shop that has done all the work on the brakes. They replaced what they thought was a faulty master cylinder, they said it was fixed. I went to pick it up, and it seems they have done nothing. The pedal still sinks way back past the gas pedal. When pumped, it will come back up slightly, and effectively brake the car. The mechanics said it could potentially be an ABS system problem. Any ideas, it isnt the typical solutions, cause they have all already been tried. Thanks.