In the position of spool 6 shown in Fig. a, port 1, supplied by compressed air from the main, is connected to port 2. Port 3, connected to the atmosphere, is closed off. When pilot air is admitted to port 5 and port 4 is connected to the atmosphere, spool 6 is shuttled to the left and held in this position by friction even after port 5 is connected to the atmosphere. At this, port 2 is connected to the atmosphere through port 3, and port 1 is blocked. The principle of the valve is shown schematically in Figs. b and c.
$3690$SHP,FC$