Compressed air, supplied from the main to port 1, passing through hole 6, is admitted through the central channel in spool 5 to the left end space of the spool, holding spool 5 in the position shown in Fig. a. At this, port 2 is connected to port 3, which leads to the atmosphere. When pilot air is admitted into port 4, spool 5 moves to the left because the right end of spool 5 is of greater area than the left end. This connects port 1 to port 2. The principle of the valve is shown schematically in Figs. b and c.
$3695$SHP,FC$