As liquid contaminants collect in the bowl, they raise a closed cell molded float. When the liquid level reaches a given point, the float triggers a mechanism, which pilots line pressure against a large-area diaphragm, which snaps open the drain valve. The contaminants are discharged from the drain orifice at line pressure. As the liquid level falls, the pilot valve closes, line pressure against the diaphragm returns to atmosphere and the drain valve snaps closed.