Abstract: A main valve has a housing, a valve member slidable in the housing and defining at its opposite ends two pressure chambers, a high-pressure port and a low-pressure port of which one communicates with one of the chambers, a throttle connecting the chambers and a spring acting in the other chamber upon one end of the valve member and urging the same from a blocking position towards a rest position. A control valve has a fluid line which communicates with the other of the chambers for selecting a main-valve response pressure. A first set of passages connects the ports with one another via the one chamber when the valve member approaches the rest position thereof, and a second set of passages connects the chambers with one another when the valve member approaches its blocking position.