The Hill-Holder, introduced on late model cars, is a device that prevents the car from drifting backwards when starting up an incline from a stop. The Hill-Holder holds the brake on, temporarily, until the clutch pedal is released and the car is in forward motion.
A Pressure Hold Valve (PHV) is connected to one of the service brake lines. A clutch connected cable opens or closes the valve. When the vehicle is stopped and the clutch depressed hydraulic pressure is maintained even when the brake pedal is released.