Abstract: A liquid level valve control system includes pressure responsive means compring a first chamber containing a first diaphragm having a port for applying pressure; bleed means enabling air to pass this diaphragm; and passageway means enabling air then to pass into a further chamber also containing a diaphragm operatively linked to the valve operating apparatus; so that, when a predetermined and substantially constant pressure is applied at the port, air bleeds past the first diaphragm into the further chamber to move the diaphragm therein to activate the valve control system, but when a surge of pressure is applied at the port the first diaphragm moves to block the passageway into the further chamber, thus preventing pressure being applied to the latter's diaphragm to activate the valve control system.