a vehicle park - brake system includes a park brake , a solenoid ; a first pressure source ; a servo for disengaging the park - brake using the first pressure source , and for disengaging the park - brake using a force produced by the solenoid that actuates first and second pistons ; and a second pressure source applied to the servo causing hydraulic pressure to vent from the servo , the pistons to detach mutually , and the servo to engage the park brake .