Brakes pedal to the floor

I have a 2000 Alero - 4 cylinder. the brake pedal was spongy and yesterday if I pumped the brake it would work, but then I would loose pressure and it would go back to the floor. Today i changed the master cylinder, because everyone said that this was probably the problem. I bled the cylinder and the brakes and the fluid is flowing, but I still have no pressure and the pedal goes to the floor. What are the chances it may be the booster? Every thing I read said that these parts usually don't go bad. I hate to have to bring it to someone. Can anyone give me recommendation? Some one said maybe it was a vacume leak because when we pumped the brake and then held, the car would start to stall.

Need Help!

I dont beleve it would stall when you pump the breaks. You already took the correct steps in trying to correct the problem, your chances of the master cylinder is a 50/50 chance. check all the hoses ans if thats not it, you could, worth a shot, call the dealership and ask what it could be. no garentee that they will tell you. you elimanated what it could be, you are on the correct path, try and replacing the master cilinder or the booster if not done so. if that does not work, then i would bring it to somebody. How are the roters and brakes? Good, bad?