Hey Speedy, are you absolutely sure the problem is the booster? I had a problem with the brakes on my 240D which I thought for sure was the booster. I replaced the calipers and bled the brakes before I tackled the booster and I'm glad I did. It solved the problem. I went from no brakes to awsome brakes that easily. Also make sure when you fill the resorvoir with fluid, you fill it to the top. On mine, it wouldn't fill the front chamber unless you basically overfilled it. But that might just be the case in my '81 W123.

Also, I'm thinking that if the o-ring between the master cylinder and the booster is bad. You would have other vacuum related problems. Such as not being able to turn the car off with the key. I also agree with rs899, those booster nuts under the dash are a bear to get to. Good Luck.