A diagnostic method to identify a leak in simulator valve in a vehicle brake system includes the steps of: (1) providing a simulator partially filled with a pressure medium and a de-energized simulator valve; (2) energizing a pumping valve, a secondary three-way valve, and a plurality of apply valves; (3) applying and retracting a plunger in a plunger assembly at least two cycles so that a predetermined pressure is achieved; (3) holding a plunger in position within the plunger assembly while maintaining a replenishing check valve in a closed/de-energized position and energizing the simulator valve; (4) obtaining a measured master cylinder secondary pressure decay; (5) comparing the measured master cylinder secondary pressure decay to a predetermined pressure decay value; and (7) identifying a leak in the simulator valve if the measured master cylinder secondary pressure decay does not match the predetermined master cylinder secondary pressure decay.