Pressure Decrease
WARNING: This page is about a different car, the 2010 Chevrolet Malibu. However, it is still accessible from the selected car via links, so may be relevant.
If a pressure hold does not correct the wheel skid condition, a pressure decrease occurs. The isolation valve remains closed and the dump valve is opened. This allows a small amount of brake fluid to escape from the hydraulic circuit into the low pressure accumulator (LPA) reservoir.