I've been familiarizing myself with my friend's '94 E320 Cabrio (117K) of late. I did his evaporator for him (yuck!) and when I started test driving it afterwards, it scared the s**t out of me when the brake pedal lost all resistance and went to the floor several times. It always stopped but barely a couple of times. It had never done this to the owner. After some research, we bet on the master cylinder and replaced it. All was well for nearly 5 weeks but last night he left me a message that it did the same thing to him. Any thoughts on other causes we may have overlooked? Is a defective new MC a possibility (ATE)? There were no visible signs of leakage on the old one.