[425HP409]

The valve in question for a drum/drum brake system is a pressure differential valve. It has a valve in it that will stay centered with equal pressure front and rear. It will move to one way or the other if one end of the brake system has a pressure drop, this will provide a ground circuit to the brake warning lamp and it will light up notifying the driver that there has been a brake pressure loss. When the loss of pressure is repaired the valve can be recentered by firmly depressing the brake pedal a couple of times.