The main valve is biased in the opposite direction from that urged by the inlet pressure, so as normally to keep it open, but the main valve snaps shut when the inlet pressure forces overcome {HEAD ~ the sum} of the downstream gas pressure forces and {TAIL ~ the biasing forces} 
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The main valve is biased in the opposite direction from that urged by the inlet pressure , so as normally to keep it open , but the main valve snaps shut when the inlet pressure forces overcome { HEAD ~ the sum } of the downstream gas pressure forces and { TAIL ~ the biasing forces }
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