Brake pedal going all the way to floor

I have a 1994 olds 88 with 34,000 miles on it. About a year ago I had the front brake pads replaced, and everything was fine until about a week ago. Sometimes, when I begin to stop everything is good, while some other times the pedal will lose pressure and go all the way to the floor and not engage the brakes well enough to stop the car. I have followed all the lines, and I don't see any leakes, I have replaced the drivers side brake cylinder ( looked like it had a lot of "sludge" built up on it), and also checked fluid level, it is good.