Abstract: An anti-theft device for a motor vehicle equipped with hydraulic brakes. A key switch is electrically connected to the vehicle's battery and to a small motor. When the key switch is turned, power from the battery causes the motor to reciprocate a piston which is connected to a valve. The positiion of the piston either blocks or unblocks the flow of hydraulic fluid between the brakes and the hydraulic fluid reservoir while one-way flow is allowed between master reservoir and brakes whether the device is armed or unarmed. The key switch and connecting wiring are isolated from the ignition system, preferably passing through the structural frame of the vehicle.