My brakes worked perfect one day and the next the pedal goes down pretty far, I put a new master cylinder and fixed a leaky rear cylinder with no difference with the pedal ive bleed all 4 wheels, the rear anti lock light comes on after driving for a little bit, but the fluid level is good. I can hear a leak from the booster, and its large enough to change the idle, but could a booster cause a low pedal?

 

Does the pedal stay firm and not sink with the engine off? If so then you have a bad booster.