Using a power bleeder, I pushed two liters of new brake fluid through the system and still had air bubbles coming out of the rear calipers. The main dealership never changed the brake fluid in ten years on this 1999, C240 and I decided to dismantle all pistons. Two had a bit of gel and one a bit of sediment. So I consider the (whole day) effort worthwhile. I never had a problem using this power bleeder (at about 20psi) on any of my other (non-ABS) cars. The front brakes had no air bubbles in no time but both rear brakes keep pushing bubbles through. Large bubbles approximately every two inches in the fuild. I eventually started the engine, pumped the pedal, but still bubbles coming through. I have since driven the car and everything seems OK. Any clues why I keep having bubbles coming through on the rear calipers? I am intrigued and may take the car to the dealer, just for a bleed - both of the brakes and my purse!