In the initial position of spool 6, shown in Fig. a, compressed air, supplied from the main to port 1, is admitted into port 2. When pilot air is admitted into port 4, spool 6 is shifted by piston 7 to the left, connecting port 2 to port 3, which leads to the atmosphere. When port 4 is connected to the atmosphere, spool 6 and piston 7 are returned to the initial position by spring 8. Channel 5 is connected at all times to the atmosphere. The principle of the valve is shown schematically in Figs. b and c.
$3694$SHP,FC$