Whatever the problem you should not be driving the car until it is evaluated and fixed...it seems quite dangerous and faulty brakes are not something to ignore.
I would check the power brake booster and master cylinder for faults. It may be something the dealer is best equipped to handle if you are not an advanced DIYer...you just don't want to mess with brakes - to me they are the most critical component for safety.
2011 E350 Bluetec Arctic White/Almond MB Tex (22K)
1984 BMW 733i
1991 Porsche C2 Cabriolet
2012 Lincoln MKz