Base Brake Valves (Normal Open)
WARNING: This page is about a different car, the 2008 GMC Yukon and 2008 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
The normally open base brake valve (N/O valve) is used to keep boost pressure from feeding back to the MC which would increase the force back to the pedal. It closes off a passage which is used to apply pressure to the wheel brakes from the MC in the case of a system failure. In this case the system failure referred to is one which results in a control mode known as push through. In normal operation the N/O valve is on (closed) during a boost pressure apply event, and is left off (open) during failed system operation.