To respond to 2 posts:

1) The brake fluid was changed back in June when I bought the car. No problems until the hard pedal. No leaks, no nothing.

2) As Benz300 said, brakes are not to be fooled with. You don't want to take advice from novices. I'll listen to ideas, but the bottom line rests with my mechanic. Again, I didn't take it to the dealership, not even a MB specialist, but a general mechanic who I think is pretty competent. He drove it after I had the problem, which occured just that one time, and the brakes worked fine for him. Shince he could really see no problem since everything was working fine, he did a best-guess estimate that the problem was the booster, and replaced it. Too bad the problem then happened a 2nd time, just a week after the booster was replaced. So this time, he replaced the hose. Hopefully all will be well.

Jeff