My guess is that your master cylinder is going. Mine did the same thing your car is doing and I had to replace it. Though I was certain it wasn't my rear brakes because I had just replaced the rotors and discs and flushed them a few weeks before the master cylinder started acting up. If it were your brake booster, you wouldn't be able to depress the pedal much.

__________________
1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver