if there is air in the system all you have to do is a complete flush. i recommend getting the motive power bleeder for a really fast way to flush the entire system.
basically set up the motive powerbleeder to the resevoir, have the car on jack stands with the wheels off, and i believe a 13 or 11 mm box wrench. then pump the pressure to 20 psi on the motive power bleeder, then start at the right rear, open the bleed screw and let the fluid flow into a bottle, do this until the fluid changed to the new color, most likely a clearish yellow. then go to the left rear, then the right front then the left front. then your done.
if its a sensor, go to dealership