C1121 00
WARNING: This page is about a different car, the 2010 GMC Sierra and 2010 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
This diagnostic detects if the left front circuit is leaking by actively applying pressure and monitoring the accumulator pressure change. The pressure change of the accumulator indicates the volume of fluid used which is then compared to expected fluid used during a brake apply. Too much fluid usage indicated a leaking circuit.