Boy, you've sure been through a lot with this mess! I'm starting to wonder about your booster. I'm not sure how a bad booster alone could cause your pedal to go flat, but it's worth a look. I forget, did it come of the junked Monte or did you get a new one?
You say you had a nice firm pedal, then you started the car and the pedal went flat. What happened after you turned the engine off? Did the pedal go firm again or did it stay soft?
Here's something to try. With the engine off and all the wheels off the ground, have somebody hold down the brake pedal and then go around and try to spin each wheel. They shouldn't budge. Then, start the engine and do the same thing. Can you spin any of the wheels? If so, which ones? This may help you find out where the problem lies in the braking circuit.