... there is no water in the pipe,the bronze valve clack drops down to the ful opening position,Air ... any air in the pipe,water cames into the valve,the valve clack goes up to the shut-off position by ... the air is released from the smal hole on lever of the valve clack,Or the air gathered at the top ... .When there is negative pressure in the pipe,bronze valve clack drops down,descharge hole is open,and then ...