Base Brake Valves (Normal Open)
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.
The normally open base brake valve is used to keep boost pressure from feeding back to the master cylinder 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 master cylinder 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 normally open base brake valve is on (closed) during a boost pressure apply event, and is left off (open) during failed system operation.