I'm with the others on this one, Ive seen master cylinders do this very same thing. But I want to ask you a question. When the brake pedal is low do you feel any vibration or buzzzing in the brake pedal? I'm wondring about ABS problems is why I am asking. It will give you a low sinking pedal feel if it is not function properly. Just hate to see you buy a very expensive master cylinder when it may not be the problem. Brake problems are something not to be taken lightly. Make sure you let us know the fix or if you need more help.