A valve having an inlet, exhaust, control port, and diaphragm. The diaphragm moves between an open position, in which the inlet and exhaust are in fluid communication, and a closed position, in which the diaphragm blocks fluid from flowing between the inlet and the exhaust. The diaphragm moves from the closed position to the open position when an inlet force due to pressure at the inlet is greater than the sum of the biasing force and a control port force due to pressure at the control port. The valve is operable to automatically drain contaminants from a vehicle air reservoir when the vehicle's emergency brakes are set. To reduce noise, the diaphragm of the valve is preferably in tension when in the closed position, and/or the diaphragm opens a limited distance when in the open position so that a back pressure is generated at the inlet.