A diagnostic method to identify a leak in a three-way valve for a vehicle brake system having a remote master cylinder which includes the steps of: (1) providing a pedal simulator having pressure medium; (2) de-energizing a secondary three-way valve; (3) retracting a dual acting plunger to the home position to drop the pressure in the boost circuit to zero while also monitoring the pressure at the output of a fluid separator via a secondary master cylinder pressure sensor; (4) determining a rate of pressure reduction at the output of a fluid separator via a secondary master cylinder pressure sensor; and (5) identifying a leak in at least one of a primary three-way valve and the secondary three way valve if the rate of pressure reduction is equal to or higher than a pre-determined rate.