Incoming pilot air at port a (Fig. a) shifts plunger 1 downward and compressed air is admitted through the port in intermediate cover 2 and the central channel in sleeve 3 to the right end of the main valve to shuttle spool 4 to the left. At the same time, air passes through a hole in plunger 1, to the space under the plunger, and fills space 5. Since the lower area of plunger 1 is greater than its upper area, the plunger is returned to its initial (upper) position after a certain time delay. Thus the right end of spool 4 is connected to the atmosphere notwithstanding the pilot air supplied through port a. After this, spool 4 remains in its shuttled position (at the left) and can be shuttled back again to the right by admitting pilot air at port b without shutting off the pilot air at port a. The principle, of the valve is shown schematically in Figs. b and c.
$3665$SHP,FC$