When there is no pilot signal S in the line, the mechanism operates as an ordinary ball-type check valve permitting air or liquid flow from a to b and preventing flow in the reverse direction. When signal S is received, piston 1, owing to the pressure difference, shifts to the left, overcoming the resistance of helical spring 3 and, with its pusher 4, moves ball 2 off its seat. The valve begins to permit air flow from b to a.
$3632$SHP,Va$